Remove assets for API controllers

API controllers are only supposed to respond with JSON. Thus, they do
not require assets.
Remove Api::RingController

We can easily add routes with the name /ring that route to the necessary
controllers
Update Factory for Tags
Implement bulk of Api::MessagesController

Also, change method name of 'send' to 'send_message' because it
conflicted with the 'send' method in Ruby.
Access messages from Tag/User in "created_at DESC" order
Use .update (i.e. <<) for hashing

The preferred way to hash multiple messages into one hash is to use
.update or <<.
Fix bug in User#send_message

The method failed because the Message was never saved. It failed
validations because it never had a tag set.