v0.3.1

Identical to 0.3.0. That version went to AMO's self-distribution channel by mistake, and
AMO rejects a version number that already exists on the add-on — measured, not assumed:
uploading 0.3.0 to the listed channel answers "Version 0.3.0 already exists." A version's
channel cannot be changed after upload and a number is never freed, so the store release
of this code has to carry the next one.

The submission page is what makes this easy to get wrong. It states the current channel as
prose ("On your own.") with a small Change link, and the file picker sits directly beneath
it — the radio buttons live on a separate /versions/submit/distribution page, defaulting
to whatever was used last, which for this add-on is every sign:dev build ever signed.
Go to that page first: ?channel=listed arrives preselected.