Revamp signin page

hi,

i've started testing the last commits on webschleuder and then i got lost in making the login page a bit more bootstrap alike. i think if webschleuder uses a bit more standardized bootstrap classes, its a win for the ux side and its a lot easier to change the look and feel of it. i hope its oke that i just change stuff- if you don't like it, just ignore the merge request 😉 (i based the changes on this example)

(the first commit doesn't really have to do with the login page...)