Add a merge migration
Add a merge migration

Merging a pull request containing
0027_make_instance_canopy_fields_non_null created a migration
conflict. I created the merge migration in this commit with the
following command:

```
$ vagrant ssh app -c 'cd /opt/app/core/opentreemap \
&& envdir /etc/otm.d/env ./manage.py makemigrations --merge'
Merging treemap
  Branch 0027_make_instance_canopy_fields_non_null
    - Alter field canopy_boundary_category on instance
    - Alter field canopy_enabled on instance
  Branch 0028_auto_20160531_1537
    - Add field searchable to boundary
    - Alter field searchable on boundary
```

maurizi/otm-core,maurizi/otm-core,RickMohr/otm-core,maurizi/otm-core,maurizi/otm-core,RickMohr/otm-core,RickMohr/otm-core,RickMohr/otm-core