Re-enable submit button in request builds overlay.

[Henning Eggers]

When requesting builds for a sourcepackage recipe, an overlay is shown to select the ppa and distroseries. Here is an example of what would happen currently:

The form offers two series
[ ] Warty
[ ] Hoary

The user selects one of them and hits submits. If it so happens that this series already has a pending bulid, there will be an error message and the form will remain open with the submit button disabled. (This only happens if the information in the form has become stale because somebody else already requested this particular build since it was opened. Otherwise the checkbox would be disabled anyway.)
[X] Warty
[ ] Hoary
* An identical build is already pending for Warty. *

The user now selects the other series to select a build for this one instead but they cannot submit the form because the submit button remains disabled. They need to close and re-open the form.
[X] Warty
[X ] Hoary
* An identical build is already pending for Warty. *

What should happen is that the checkbox should be disabled and the submit button enabled as soon as the user selects another distroseries to request a build for.