* Changes the headings' rank of the security settings section

This commit changes the existing headings' rank of the security settings section from level 6 to level 4.

* Renames the auth.change_password string into auth.security

The "Security" preferences' section used to be called "Change password". When it was renamed, the string name wasn't changed.
This commits changes auth.change_password to auth.security.

* Adds a heading to the password change form

There was previously no heading for the part of the "Security" page that contain the password change form.
This commit adds a rank 4 heading to this section and reintroduces an "auth.change_password" string to be used inside it.

* Removes useless HR elements

The various sections of the "Security" settings page were previously separated by HR elements.
Now that there is proper headings, they're not required anymore.

* Updates CSS

This commit updates CSS in such a way that the same style is applied to all the H4 elements of the settings.

* Correct a mistake

A character went missing on one of the previous commits, broking the CSS.
This new commit fixes it.

* Redesign landing page (again)

* Move login form in small version to the right column

* Display closed registrations message

* Add site setting for the hero image

* Fix test

* Increase spacing, maximum width, change call to action section

* Remove onboarding modal

* Welcome e-mail

* Send welcome e-mail after confirmation

* Remove obsolete translations

Serbian latin (sr-Latn) is generated automatically from Serbian (sr) translation. Also changed some wording in original (Serbian) translation.