Implement Poniverse display name syncing

[EventideGlow]

Pony.fm has a "sync my display name with MLP Forums" checkbox in the account settings that doesn't actually do anything right now.

The option should be changed to "sync my display name with Poniverse" and the feature should be implemented.

[EventideGlow]

Food for thought: is this dangerous, considering the possibility of breaking links to your Pony.fm profile by changing your Poniverse display name? Maybe slugs shouldn't be tied to display names? Or we should set up redirects from old slugs?

Feedback plz.

[kz]

@Feld0 - Have you considered migrating away from using display names as IDs?

Here is a possible migration workflow:

1. Create a legacy_display_name column which copies all users' display names
2. Set up a new URL such as /user/{id}-{slug} or /user/{id}/{slug}, where -{slug}//{slug} is optional, although links to users will include the slugs - the former is how IPB does it (https://mlpforums.com/user/36898-raynemercuryTESTASDFGHJKL/)
3. If a legacy URL (/{legacy_display_name) is hit, then the user will be redirected to the new URL.

[EventideGlow]

@kz "Pretty" URL's for profiles are worth the weirdness - SoundCloud, Bandcamp, deviantART, Twitter, and GitHub all manage with them at a much larger scale than Pony.fm. We won't abandon this URL format for profiles in the foreseeable future.

We don't currently set up any redirects when someone changes their display name - I only raise this in connection with Poniverse name syncing because it may cause Pony.fm display names to change much more often.

[LavaToaster]

FYI, this documents what Github does when changing the username.

It sounds like requests to a resource are made first, and then a backup redirect is checked after should the initial fail.