Unifying actions 'join' and 'auth' and breaking 'join' in blocks

Now there's a single action to join meetings: 'join'. It can be called via
GET or POST, and accepts the parameters as previously accepted by 'auth'
(user's name and password).
Also, 'join' is now broken in several smaller methods (before filters) to make
it simpler.

This was all done to make the join process in mobile devices better. We need
to display the 'join_mobile' view as an intermediary step when joining from
mobile devices. And it needs to be done when joining directly from 'join' or
from 'invite', and previously was done only for 'join'.

Not finished nor tested yet.

refs #1247
Add link to the mobile application in the Apple Store

And made the "download from x store" badges local images, not external URLs.

refs #1247
Simplified redir_url param in the join_mobile action

Use only the path, not the entire URL, to make it a bit cleaner.

refs #1247
Fix tests after changes for #1247

refs #1247
Update Google Play badges

refs #1247