Can't use actual extras and `convert_to_extras` fields when using a custom schema

**TL;DR** You could not use actual extras and `convert_to_extras` fields when using a custom schema because the `convert_to_extras` function was not working as the validation expected.

**Update**: This now also includes logic to prevent using a free extra that has the same key as a field from the schema (which will allow to bypass validation)

--

Take the following custom schema implemented with `IDatasetForm` (this is similar to the one used on the `example_idatasetform`):

    class MyPlugin(p.SingletonPlugin, p.toolkit.DefaultDatasetForm):

    # ...

    def create_package_schema
        schema = super(MyPlugin, self).create_package_schema()

        schema['custom_text'] = [ignore_missing, convert_to_extras]
        schema['custom_text2'] = [ignore_missing, convert_to_extras]

        return schema

    # ...


If I call `validate` with this schema without any extras, all works as expected:

        data_dict = {
            'name': 'test-dataset',
            'custom_text': 'Hi',
            'custom_text2': 'Hi2',
        }       

        data, errors = validate(data_dict, schema, context)

        '''
        {'custom_text': 'Hi',
         'custom_text2': 'Hi2',
         'extras': [{'key': 'custom_text', 'value': 'Hi'},
                       {'key': 'custom_text2', 'value': 'Hi2'}],
         'name': u'test-dataset',
         'title': u'test-dataset'}
        '''  

One could argue that the fields should not be on the root any more, but that's another matter, we got them on the extras, which is where we needed them to get them stored.

Now, if we provide some "actual" extras in the data_dict things start to fail:

        data_dict = {
            'name': 'test-dataset',
            'custom_text': 'Hi',
            'custom_text2': 'Hi2',
            'extras': [
                {'key': 'proper_extra', 'value': 'Bye'},
            ]

        }

        data, errors = validate(data_dict, schema, context)

        '''
        {'custom_text': 'Hi',
         'custom_text2': 'Hi2',
         'extras': [{'key': 'proper_extra', 'value': 'Bye'}
                       {'key': 'custom_text2', 'value': 'Hi2'}],
         ],
         'name': u'test-dataset',
         'title': u'test-dataset'}
        '''

For every actual extra that you provide on the data_dict, one of the fields that were supposed to be converted to extras will get ignored and not put in `extras`, which means that they won't get stored (and validation will pass even if you were using `not_empty` or similar).

Even worse, if there are more actual extras than `convert_to_extras` fields the dictization will actually raise an exception:

        data_dict = {
            'name': 'test-dataset',
            'custom_text': 'Hi',
            'extras': [
                {'key': 'proper_extra', 'value': 'Bye'},
                {'key': 'proper_extra2', 'value': 'Bye2'},
            ]

        }

        data, errors = validate(data_dict, schema, context)

        '''
        File "/home/adria/dev/pyenvs/glasgow/src/ckan/ckan/lib/navl/dictization_functions.py", line 235, in validate
            converted_data = unflatten(converted_data)
        File "/home/adria/dev/pyenvs/glasgow/src/ckan/ckan/lib/navl/dictization_functions.py", line 399, in unflatten
            current_pos = current_pos[key]
        IndexError: list index out of range
        '''

The relevant code to understand what's going on is located in [dictization_functions.py](https://github.com/ckan/ckan/blob/master/ckan/lib/navl/dictization_functions.py#L233-L235), in the `validate`function:

        flattened = flatten_dict(data)
        converted_data, errors = _validate(flattened, schema, context)
        converted_data = unflatten(converted_data)

If we look at `converted_data` just after calling `_validate` when validating a dict against a schema that uses `convert_to_extras`, we see that these extras are not added in a "flattened" way, but in an `('extras',)` key:

        {('custom_text',): 'Hi',
         ('extras',): [{'key': 'custom_text', 'value': 'Hi'}],
         ('extras', 0, 'key'): u'proper_extra',
         ('extras', 0, 'value'): 'Bye',
         ('name',): u'test-dataset',
         ('title',): u'test-dataset'}

When "unflattening" things, `unflatten` will replace the n first elements of the `('extras')` list with the n actual extras that were provided.

So it's pretty obvious that the culprit is [convert_to_extras](https://github.com/ckan/ckan/blob/master/ckan/logic/converters.py#L10) itself, which is not adding the new extras in an unflattened way, which is the one expected later on.

After applying the patch on this PR, this is how `converted_data` looks like before unflattening:

        {('custom_text',): 'Hi',
         ('extras', 0, 'key'): u'proper_extra',
         ('extras', 0, 'value'): 'Bye',
         ('extras', 1, 'key'): 'custom_text',
         ('extras', 1, 'value'): 'Hi',
         ('name',): u'test-dataset',
         ('title',): u'test-dataset'}

Which gives the expected:

        {'custom_text': 'Hi',
         'extras': [{'key': u'proper_extra', 'value': 'Bye'},
                    {'key': 'custom_text', 'value': 'Hi'}],
         'name': u'test-dataset',
         'title': u'test-dataset'}