v0.13.1

A small one, about the first upload and about what the upload button promises. Two things could go
wrong quietly before, and both now say what they are going to do before you click.

Publishing

Bug fixes

• The target language can no longer be changed on the first upload. It was prefilled and still
  open: picking another one made the site list the translation under a language the file was not
  in, and the next launch reported a language conflict on a translation you had just published.
  The target settles with the first translated line; the setup screen now shows it as settled and
  only asks the source.
• "Contribute" is no longer offered on a translation whose author works alone. The button said
  Contribute, the upload screen opened, and the site refused after the upload. When the only way on
  is Fork, the button says Fork and forks — the same confirmation as the Fork button of the role
  row. The same goes for a translation whose Main is gone or whose author erased their account.
• The upload screen no longer forgets that a Main takes no contributions. Opening it cleared
  what the sync had learned, so the "Contribute as Branch" row that had been hidden came back on
  the next refresh.

Shared rules

• What an upload becomes — create, update, contribute or fork — and which language pair a file
  may be published under are now decided once, in the shared library, and the Manager reads the
  same rule. A file that states another pair than its published lineage is refused with the same
  sentence in both.