Dev UX Improvements

[geoffturk]

Changes to API to make it more developer-friendly

• GET /categories should return an array of objects (name and id) instead of an array of strings (name) so that PATCH/DELETE /admin/categories/{categoryID} can be called to update/remove categories.
• Same as above but for GET /tags.
• Add remainingMaximumPositiveBalance and remainingMaximumNegativeBalance to GET /user/entities/{entityID} (shows how many more credits can be received or sent without having to calculate it from current balance).