Use ActiveRecordStore as session store

When saving a proposal fails due to JSON validation errors (missing mappings / options / typos), Crowbar presents a flash message with the JSON parsing error. However, messages like "743: unexpected token at '{234 "use_virtualenv": false,..." may easily cross the 4k border that the default session store (CookieStore) can handle. In these cases, Crowbar just bails out with a 500.

The only solution is to set a different session_store. MemCacheStore would probably work too but needs more refactoring due to more funny session object fiddling inside ApplicationController.rb. Therefore it's probably best to use good old ActiveRecordStore. However, this re-introduces the dependency on ActiveRecord and it needs a database. The default sqlite3 should be sufficient since we don't expect many users to use crowbar at the same time.

Here's an example backtrace: http://pastebin.com/raw.php?i=dJDMUYr9