Create wikis in a background process

[edg2s]

• The form now points to start.php, which converts allowed POST data to environment variables and sends them to new.php
• The output is written to log files in log/*.html which are then polled by the JS for new content every second
• Logs are only deleted when the wiki is deleted, allowing for debugging and inspection by other users.

Fixes #399

[edg2s]

Possible future enhancements:

• Keep logs of failed wikis around for a certain amount of time for debugging.
• Add the ability to abandon a wiki creation manually.
• Show wiki creation state in the public list with a link to the log visible to all users.