[stills] Still generation form

[robert]

Create a still where the user can start still generation. The location of this form is a bit uncertain, most logical place is the edit page of the media.

Still form of CSA must be re-used, as it was re-used in the D6 version as well.

The still form should be (re-)used in 3 places;

• The edit page where you can re-generate the stills.
• The upload page, where the still form is hidden by collapsible item.
• The settings page, where you can define the default settings when user is uploading.

[forgacs]

The admin settings for still generation created.
There is a new permission now.
Upload form created.
Edit form is on a new tab near the edit tab.
All code is in the mediamosa_ck_still module.