Creates a DocumentTag model for handling document tags

The DocumentTags will be pre-seeded, and Documents will be able to have
many tags.
Creates join table for DocumentTags and Documents
Creates relations between Documents and DocumentTags
Lists tags in Select2 box on document edit page

Saving currently failing as the Document does not accept nested
attributes for tags.
Document tags are editable
Adds preset tag seeds
Uses STI to give types to DocumentTags
Squashes migrations together

No need for multiple migrations for such simple tables
Removes unneeded pending tests
Merge branch 'documents-tags' into develop