Adding anonymous account support

* Added anonymous flag to accounts table
* Added checkbox for anonymous flag in edit account page
* Updated user class to support new flag
* Updated statistics class to support anonymous and donations
* Updated all templates showing usernames to show anonymous instead
* Added new SQL `ALTER TABLE` file for upgrading the table

Fixes #419 once merged.
Merge pull request #454 from TheSerapher/issue-419

Adding anonymous account support