Issue #147 - Gracefully handle non-existent tags

[hilarysk]

What does this PR do?

1. Adds a tag index page
2. Adds a link to the tags index page in the header
3. Adds tags to the db seeds file
4. Adds a test for the new controller action
5. Adds new factory for tags
6. Adds a note on the tag show page if there aren't any proposals currently associated with that tag

Why was this work done? Is there a related Issue?

Addresses Issue #147

Where should a reviewer start?

Probably the Tags Controller

Are there any manual testing steps?

Run the new seeds (note: this is destructive; if you have local data to keep, you can manually run the tag creation section of the seeds file). Navigate to "Tags" from the header.

Screenshots

Deployment instructions

N/A

Database changes

N/A

New ENV variables

N/A