Now, on OAuth signup form, we create a unique username with random appended string only if there's a conflict.
Previously, this was always happening during the Slack OAuth flow. This has the benefit of preventing username collisions for all OAuth providers.
This adds any OAuth login buttons to the invite signup page, stores the invite code for the flow duration, and associates the new user with it once successfully registered.
It enables invite-only instances with OAuth-based registration.
Having a `Username` field populated in the page data tells the base
template to display navigation that only a logged in user should see. So
this renames the field to `LoginUsername`, similar to our login.tmpl
page.
Ref T712
