Add username changing functionality.

This is an incomplete implementation of username changing code.

The things still needed to be finished:

* The method that changes the member attribute has to be filled in.
* The functionality needs to be tested.
* Unit tests need to be added.
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f