Auto room creation

Adds an "auto room creation" admistration option, which makes it so if a user loads jabbr with a room url for a room that does not exist, it is added automatically. This allows for dynamic urls to be used that invite users to discuss a topic without having to make the room in advance. The setting is disabled by default.

Note that this PR builds on top of my previous PR, #1078 because of changes to the administration index.cshtml. Recommend you take that one first if you are going to take both of these.