This commit implements a text field where user can edit overrided display names for other users.

Type: add

CCBUG: https://iron.lily-is.land/T77

• Verify that unit tests pass.
• Open a user page of some other user. Observe that there is a text field "Custom display name...". Input something into it and click "Save". Verify that the user's display name is updated in other places (see https://iron.lily-is.land/D38 ).
• Open your own user page. Verify that no such text field appears.