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:

• 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="one-time-code" on TOTP code input fields (behaves the same as "off" in Firefox/Chromium)
• autocomplete="username"/"email"/"nickname" on login/signup/selfservice inputs wherever appropriate