Remove unnecessary message composer object

Since we no longer need to store the response time of an agent or
calculate the average response time in general, we can switch back to
newing up records the normal way for now.
Remove unused outstanding messages query object
Remove column from message to record time of response
Display time of most recent message
Sort conversations by most recently added message

The implementation for this is pretty bad and needs to be improved. We
could either do the sort as part of a SQL query or update a
`message_added_at` column to the conversations table that is updated for
each message added.
Use helper to color the most recent message time
Apply styles to most recent message times
Display message updated timestamp
Add flexmock assertion heleprs to minitest specs
Create an object to search/query for conversations
Move conversation querying logic out of the controller
Cleanup whitespace
Merge pull request #81 from vanstee/conversation-response-time

Display conversation response time