Login: Render auth providers as round buttons (still ugly)
CSS: Refactor work package and account stuff into own files
Center main login form and use generic form formatting

No more table!
Login forms: Improve looks
Main login form: Add accidentally removed "stay logged in" option
Pulldown login form: Improve looks

Also contains some small improvements for main login form.
rename of labels to get tests running again
removal of colon to ensure capybara is able to find input
removal of second colon to get tests running again
rename of label label_register to get tests runnning again
Merge pull request #1262 from opf/feature/multi_provider_login

Multi provider login forms