Update calendar.
Addincremental upgrade path from 1.2 to 1.2.1 on the alpha channel.

This will make the manual testing of the incremental upgrade (for
Tails 1.2.1) a lot easier.
Updating trace file after uploading 1.2.1.
Merge branch 'stable' of https://git-tails.immerda.ch/tails into stable
spelling/grammar
Merge remote-tracking branch 'origin/master' into stable
Merge remote-tracking branch 'bitingbird/stable' into stable
Update upgrade-description files for the new 1.2_to_1.2.1 IUK.

The old one lacked critically importat Tor Browser files due to a bug
in the IUK generation code, fixed in tails-iuk commit 6b89b81.
Fix syntax.
Clarify that the workaround only applies after upgrading.
Update website PO files.
Add dummy changelog entry for Tails 1.2.2.
Explain how to exchange files with the I2P Browser (Closes: #8288)