...and add a missing CHANGELOG entry.

So far, the metrics module discarded all requests for vanilla bridges as
invalid because the function isTransportSupported() only returned True
for PTs.  This match renames the function to isBridgeTypeSupported() and
makes it return True for vanilla bridges.

This patch fixes <https://bugs.torproject.org/32203>.

See CVE-2019-16865 for more details:
<https://nvd.nist.gov/vuln/detail/CVE-2019-16865>

BridgeDB uses the binary dkimverify in /home/bridgedb/.procmailrc.  The
binary used to be provided by the python-dkim package (which is
installed on polyanthum) but with the upgrade to Debian Buster, it is
now provided by python3-dkim (which is not installed).  This patch makes
this OS-level dependency explicit.

This commit fixes <https://bugs.torproject.org/32105>.

Also, add instructions for our translators.

As far as I understand the process, it's not necessary to run
'update_catalog' when requesting new translations.  We're requesting new
translations by *only* updating bridgedb/i18n/templates/bridgedb.pot
because it's the only file that Transifex pays attention to.

Once our translators did their job, we're pulling the updated,
language-specific .po files using the script
maint/get-completed-translations.