Snippets (or rebranding them to tags)

[No767]

While reading the docs for modmail, I picked up this really nice feature. Essentially it's a small piece of text that can be used to send to the user instead.

This feature contains three functions:

1. Creating, Editing, and Deleting tags. Basically this should be straight forward to deal with. R. Danny has an awesome tags cog that we can use to base the code off of
2. Listing, and aliases for snippets (basing it off of rdanny)
3. An option to view it without sending it to the thread (probably an option or flag would be best)