Misc fixes to UploadSourceField

Follow-up I7c15fd4250 (00ca365).

- Added 'radio-id' configuration option to set the id of the radio button,
  and changed Special:Upload to use it. Had to keep the old way to set it
  since this class is used by some extensions.
- Always set the "for" attribute of the label to the field and the not the
  radio, since the text it contains does not describes the radio button.

Change-Id: I524f8743fd03546024f8c97852a2bcb3c23c6019
Merge "Misc fixes to UploadSourceField"