User should be able to change usernames

[holmesworcester]

Right now users can't change their username, but now that we have an "unregistered" status there's no reason to impose this limit: users can switch names to something unregistered and request it from the registrar.

Then the "username taken" flow (#1697) when a user attempts to register an existing username gets simpler: we just have to show an alert and let the user continue to the flow for choosing a new username.

We should put the ability to change usernames in the user's profile.

NOTE: When someone's username changes, we should ALWAYS show a message:

"@oldusername has changed their name to @newusername"

[jgaylor]

@holmesworcester where do you expect to see this message "@oldusername has changed their name to @newusername" ?

Some possibilities:

1. Within the chat where you both are members (DM or Channel) a system message might be posted stating the old and new usernames.
2. Push notification informing you of the change?
3. Other?

I wasn't able to find any examples of this. If you have any, it would be helpful

What are your thoughts?

[jgaylor]

What if they change it multiple times, should we show a history or just the most recent one?

[holmesworcester]

So far we're putting stuff like that in the general channel, which is where we put the 'joined' message.

[jgaylor]

Here's the latest Add Figma and Loom