Add django-debug-toolbar

As it can do SQL explain and many other magic things while browsing the
API.

Related to https://www.pivotaltracker.com/story/show/76514752 [#76514752]
Tweak gitignore
Merge pull request #64 from sealevelresearch/minor-gitignore

Tweak gitignore
Turn on django-debug-toolbar on EMERGENCY_DEBUG

So we can use it on Heroku staging.
Merge pull request #63 from sealevelresearch/add-django-debug-toolbar

Add django-debug-toolbar
Put django-debug-toolbar in common requirements

So that it can be used on staging.
Merge pull request #65 from sealevelresearch/fix-django-debug-toolbar-on-production

Put django-debug-toolbar in common requirements
Read INTERNAL_IPS from environment

So that django-debug-toolbar will actually display.
Merge pull request #66 from sealevelresearch/read-internal-ips-from-environment

Read INTERNAL_IPS from environment
Finally fix django-debug-toolbar on Heroku staging
Merge pull request #67 from sealevelresearch/fix-debug-toolbar-on-staging

Finally fix django-debug-toolbar on Heroku staging
Add database index on Minute.datetime
Add database model CombinedPredictionObservation

This uses a custom SQL view written in the initial migration. The Django
model is "unmanaged" meaning it doesn't try to modify the database.

[#75795976]
First pass at /sea-levels/ endpoint

Much of this has been taken from the Prediction app. It uses very
similar view and serializer pattern.