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.

Same style note as in: https://github.com/dpla/KriKri/pull/25/files#r21333772