[Tags] Add ability to style tags with background images

_From @tobscure on August 27, 2015 13:49_

The database fields are already in place for this – they just need to be made use of!

What needs to be done:
- Update the CreateTag and EditTag command handlers to allow saving of the backgroundMode property.
- Add an API endpoint for uploading a background image (use core's avatar uploading stuff as inspiration).
- Add the appropriate fields to the admin JS extension's EditTagModal. The backgroundMode field should be a dropdown with the options repeat/no-repeat.
- Update the TagsPage to style tiles with their background images/modes.

_Copied from original issue: flarum/tags#13_