Removed registration form, cleaned up login flow (#3821)

Fixes #3813. Main changes:

- [x] Got rid of the registration form in favor of using the `/auth/`
view. Having two places where the user can potentially register is
confusing.
- [x] Cleaned up different aspects of the login flow that were mentioned
in #3813, like inconsistency in terminology (`Login` vs `Log in`),
removal of buttons where they don't need to be, etc.
- [x] Added a button on the password-ed login to bring the user back to
the passwordless log in/register view.
- [x] Have buttons in column rather than row on *both* mobile & desktop
view and made them the same width (`18 rem`).
- Should we center the login page for desktop? This was done as it felt
awkward to have everything on the left side while the buttons crept to
the center
- [x] Added icons to `Log in with <ORG> email` & `Log in or register via
email` for consistency
- [x] Moved login buttons to their own templates as some were being
reused
- [x] Added missing translation blocks