Make the submitted file version 1 when a preprint is accepted

[ajrbyers]

When a moderator accepts a preprint that has no version yet, the submitted file now becomes version 1. Before, accepting showed a warning and the moderator had to turn the file into a version by hand first.

What changed

• Preprint.make_first_version() makes the submission file version 1 if the preprint has no versions. Accepting calls it; an existing version is left alone.
• The Accept button is shown when there is a submission file, not only when a version exists.
• The callout on the preprint page explains that the submitted file becomes version 1, and how to use a different file instead.
• repository/tests/test_first_version.py checks that accepting a preprint with no version makes its submitted file version 1.