Dialogs: Make UI view the same for both dialogs

1. Go to JCommune as anonymous
2. Click Sign in / Sign Up

AR:

• Couple of fields of Sign up dialog have 2 captions: inside and above field
• Button size is different

ER:

• Fields captions on both dialogs are the same: text is placed inside of the field
• Button size and alignment on the dialog are the same for both

Note: Error messages are also expected to look the same way for both
It cannot be checked now because of JC-1081