Styling for the Signup modal plugin

Acceptance

[x] ~Signup Modal should be bigger, since we have a lot of fields there~ It auto-resizes
[x] Remove the word collective from the checkboxes
- [x] Test
[x] (pending discourse pr) Remove the wrapping span and div elements
[x] ~Create a form-group called Collectives~
[x] (pending discourse pr) Move password field where the default position is (after Name or after Username)
[x] Fields should render with the same classes as Discourse vanilla, so our theme works with these fields