Add and modify institutions and harvest sources

This changeset is a preliminary step towards starting an initial harvest:  https://www.pivotaltracker.com/story/show/82567548

I am adding resources for creating, modifying, and deleting institutions and harvest sources.

Specifically:
* I have filled in `HarvestSourcesController` and `InstitutionsController`, and their related views.
* I have added new routes for these controllers.
* I have added validation of the `uri` field to the `HarvestSource` model.
* I have added tests for the `HarvestSource` and `Institution` models, and have augmented the test for `HarvestSourcesController` to check that feedback is given for invalid input.

All of the controllers are HTML at the moment.  There's no provision yet for JSON.

It seems that it should be possible to do this redirect without the find query above, just using institution_id.  I'll look into that.