Created form for subtitle tasks with no language set.

This handles the case where we link to the task, but we can't send the
user directly to the editor because we need to set the language for
the task (and also possibly the primary audio language code for the
video).
Refactored create subtitles form.

Generalized the DashboardCreateSubtitlesForm as
MultiVideoCreateSubtitlesForm.  The new code handles the same stuff as
the old one, plus it:
  * Disables the input for primary audio language if it's already set.
  * Disables the select options for languages that are already
  created.

Moved common code from CreateSubtitlesForm and
MultiVideoCreateSubtitlesForm into a base class.

Added CSS styles for disabled inputs for our forms.

Updated forms tests a bit.  Refactored templates a bit.
Replaced task links on the dashboard and tasks pages

Made team forms that subclass CreateSubtitlesForm and
MultiVideoCreateSubtitlesForm, and also handle tasks.
Made Edit Subtitles link on the language page load the new editor
Fixed task links on the user profile page
Removed "This video doesn't have any subtitles yet" text

We were showing it all the time, regardless of the status of the
subtitles.  Like I said in 1121, I don't think we need this text at
all anymore.