Revert "Remove styles for #mw-createaccount-join"

The CSS makes the button not full-width and gives it a white border to
stand out from the image behind.  These will be unnecessary when we
right-align Agora buttons and drop the background image, but not yet.

This reverts commit c6218b8f008403c519d797140b76ae4f215347e3.

Change-Id: I39776b554909978094bfaa031b3ece6066f998ca
Merge "Revert "Remove styles for #mw-createaccount-join""