Update front end files to working status with the database

With this PR, the extension is now working (if you manually add some text to the DB to test it)!!! :smile: 

This PR contains the migrations that update the database.
The announcement data are stored in the config_text table.
The user's status is stored in the user table as a simple boolean (1 shows ann... 0 if they closed it).

The logic for getting the announcement data and displaying it is all in the listener. I assume that by making this a much simpler example of an extension, this would be OK, as the logic is quite simple and will only be re-used once again, in the ACP.

Note: My idea for displaying announcements is simple: On installation, the user status bool is 0, so nothing shows. Any time an announcement is submitted in the ACP, all user status bools get switched to 1 so the announcement will show. When a user closes the announcement, the user's status bool goes back to 0 and they don't see it again, until the next time an announcement is submitted in the ACP, switching all bools back 1.
If you're ever bored and want to do something in phpBB at some time, I might suggest copying the config tool and making one for config_text which does the same thing. :)