Authentication visual nits

[lampholder]

Stuff worth fixing:

• 'Email' field has focus when you first open registration, should be 'username' (Auto-focus username on registration matrix-org/matrix-react-sdk#2546)
• Phone field placeholder is truncated
  
• strange behaviour on emptying the server field and tabbing away
  

More trivial stuff:

• https://matrix.org is an odd placeholder for modular since it's almost the one value we never expect anyone to put into this field
• On both the modular and advanced flows, I didn't see the edit option to change the homeserver details because I was looking for a 'back' button (as a counterpart to the next button that brought me here)
• auth fields nudged out of place by password manager UX (not suggesting we fix this)
  
• I get a scrollbar on the slightly-taller matrix.org registration page which I don't get on the other pages, so the centralised main box nudges left and right as I switch between matrix/modular/advanced

[lampholder]

Just making a quick note here of the things I tested which worked:

• registering on a HS which does not support msisdns
  • the phone number field did not appear
• registering on a HS which does not support registration
  • an error message appeared before I tried to register (though the fields were still enabled, which could be better)
• registering on a HS which requires email address
  • the "(optional)" text was removed from the email field placeholder

[lampholder]

I think all of these issues have since been addressed