This removes a bit of friction.

Ref T715 T712

Ref T712

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

This checks the user's inputted username as they type it, and prevents
form submission if the name is taken.

Ref T712

This includes:

- A new `user_invites` config value that determines who can generate
  invite links
- A new page for generating invite links, with new user navigation link
- A new /invite/ path that allows anyone to sign up via unique invite
  link, even if registrations are closed
- Tracking who (of registered users) has been invited by whom

It requires an updated database with `writefreely --migrate` in order to
work.

This closes T556

- Add server-side validation
- Normalize usernames
- Check username existence when signing up
- Tailor UI to whether federation is enabled or not
- Return user to form and show errors on sign up