[#16] Improve EC API handling

Set up properly the validators for them in the schema.

Also, for resources, get automatically the DatasetId from the parent
dataset if not already defined on the resource dict.
[#19] Handle file uploads on action tests
[#19] Hide Link option from the upload field in resource form

This also makes the uplod field be rendered by default, which is needed
for the functional tests.
[#19] Add functional tests for file creation, including uploads
Fix DetachedInstanceRevision in tests

By moving all calls to `helpers.reset_db()` to the `teardown` methods of
the test classes instead of the `setup` ones.
[#28] Display custom metadata in dataset and resource pages

This can be polished later on. In the resource case some custom fields
can probably be matched against default core ones, like creation_date.
Merge branch 'master' into 45-ec-harvester