consistent use of form-label class for all form labels

[stdweird]

Right now only the file attachment widget uses the form-label class and it is hardcoded (see apps/dashboard/app/views/batch_connect/session_contexts/_file_attachments.html.erb).

It is nice, but annoying atm because it is only for the this one widget and thus eg the support page looks inconsistent.

Ideally some default per form or global default can be used to toggle this on or off, but per widget setting is also ok now.