Add ability to modify full/display name in user.profile after account creation

It would be nice if we were able to update our full/display names after creating an account for consistency, privacy, and other reasons.

For example, my legal name does not match my name on Keybase any longer, and I would prefer it did.

Note - I think adding the ability to change your username should be a separate request, and is not what I'm asking for here.

If there's a better place to submit this issue or I'm missing something, let me know.

The issue identifies user.profile as the requested area but names no files, tests, or implementation entry point. Start by locating how profile and display-name data are stored and updated, then determine the validation and authorization behavior needed to support changing the full/display name without changing the username; done means the account can update those names after creation and the behavior is covered by tests.

Written by the indexing model from the issue text.