cms

* Update user interface to support joining to contests

There are 2 tabs, one for creating new user and one for register
with an existing account. Note that the register page sould be
scrollable because of the long form inside it. So it can not use
login_box class.

* Implement logic of join contest without user creation

* Add contributor's name and copyright

* Check participations only when joining a contest

* Disable and hide register fields when joining to contest

The previous method had 2 different forms but the new method has a single
form and just disables extra fields if they're unused

* Clear field errors after each tab switch

* Small updates

Co-authored-by: Andrey Vihrov <andrey.vihrov@gmail.com>