Fix autocomplete behaviour in Firefox

Firefox autofills all type="password" inputs with passwords from its built-in
password store. This breaks usability of admin pages.

This change fixes that by adding autocomplete="new-password" to these inputs.
It also adds appropriate autocomplete attributes to other forms/inputs to
improve autocomplete behaviour across browsers:

- autocomplete="off" on all non-login/signup/selfservice forms
- autocomplete="new-password" or autocomplete="current-password" on all
  type="password" inputs to workaround Firefox's misdetection
- autocomplete="username"/"email"/"nickname" on login/signup/selfservice inputs
  wherever appropriate
- Avoid type="password" where possible (e.g. on readonly fields)