Login page

[rafalp]

Adds login page that works without JavaScript

TODO

• Make "Sign in" button in navs link to login page.
• Add social auth buttons
• Add "forgotten password" link
• Return 404 if LOGIN_URL is not current url (means site owner replaced login page with custom one)
• Display banned page if user is banned
• Display sign in fallback page if auth is delegated
• Add decorator that renders login page for anonymous users. Support custom header message, redirect back to the page after successful login.
• Support next arg in POST and GET
• Make login links in emails point to new login page
• Do only redirect to next page if user is already signed in
• Add template outlets above and below login form
• Make view responsive
• Write tests for auth checks
  • Account is inactive (user activation)
  • Account is inactive (admin activation)
  • Account is deactivated
  • Account is banned (admin account)

Fixes #544

[coveralls]

coverage: 97.456% (-0.02%) from 97.477%
when pulling 41f9307 on fix-544-add-login-page
into 7109be4 on main.