Add usercontent template tag to hide data

On the user profile page we need to hide the personal data and edit
controls from the public but show them to the logged in user.

The main benefit of the tag is to make template code that shows
non-public data to only one specific user more self documenting.
Rename audit_extras to auth_extras

Both template tags deal with authorization rather than auditing.
Lint
Add a JSONField that translates db strings to python via JSON

Just make a JSONField field:

```
class MyModel(models.Model):
   stuff = JSONField()

...

m = MyModel()
m.stuff['user'] = {'age': 4, 'name': 'Peggy'}
m.save()

```

JSONField does not currently support lookups.
Add config JSONField to instance

Also provides a way to create simple getters/setters. If we want to add
a new top-level setting simply run:

```
advanced_search_filters = _make_config_property(
   'advanced_search_fields')
```

Which will allow you to write and read that property directly from an
instance:

```
i = Instance.objects.get(pk=?)

i.advanced_search_fields = ['field 1', 'field 2']
i.save()
```
Merge pull request #196 from jwalgran/topic/usercontent-tag

Add usercontent template tag to hide data
Merge pull request #206 from ahinz/topic/instance-config

Topic/instance config
Add utility functions for setting and getting visited instances
Use last instance visited for non-instance "Explore Trees" link
Made utility function names consistent
Merge pull request #207 from maurizi/topic/storeInstances

Topic/store instances
Merge branch 'master' of github.com:azavea/OTM2