[#322] Fix styling for RSR signin page form fields

Not perfect, but better than before. Needs either better CSS knowledge
or a much larger refactoring (probably both) to make it look really good.
Merge branch 'develop' of github.com:akvo/akvo-rsr into feature/322_rsr_signin_form
Merge pull request #468 from akvo/feature/322_rsr_signin_form

[#322] Fix styling for RSR signin page form fields