Allow username modification.

[RokeJulianLockhart]

Request

Please permit users to modify their usernames at forum.qt.io.

Rationale

When people modify their names by law, or replace their online pseudonym, they may wish to have their account at forum.qt.io reflect that.

Considerations

1. Because this is already possible when performed by a moderator, there should be no technical limitation.
2. If database thrashing is a concern, limit the time period between modifications.

[RokeJulianLockhart]

Posted to QTWEBSITE-1222, per post/820508.