Versions bump and Changelog update

Fix a build reproducibility issue caused by different permissions on
bookmarks.html.

boklm authored Dec 16, 2019 and Matthew Finkel committed Feb 05, 2020

Create a file containing firefox platformVersion and buildID.

This updates the version of the pion webrtc library used by snowflake
to v2.1.18.

Geckodriver is not built by default in release builds. However, it's useful
if we build it reproducibly so we can avoid trusting some binary downloaded
from the Internet for our QA and Selenium related work.

Only the 64bit Linux version is built and included as I doubt we'll need
geckodriver on anything else for our purposes.

Toke Høiland-Jørgensen generously offered to set up a default bridge in
Denmark.

This fixes <https://bugs.torproject.org/32891>.

Changelog update and versions bump

When creating update responses for nightly builds, we don't want to
create a downloads.json file, so we make that optional.

Namecoin will be enabled if all the following are true:

* rbm var/namecoin is set to 1 at build time
    * This is true for nightly on linux only
* env var TOR_ENABLE_NAMECOIN=1 is set at run-time

var/set_default_env is changing $HOME, preventing gpg to find the keys
in its default configuration directory. We avoid that by reseting $HOME
to its previous value before running gpg.

Update testsuite_git_commit so that incremental mars are generated
as part of our nightly builds.

In rbm.conf we currently have var/max_torbrowser_incremental_from set to
2. In order to be able to generate incrementals from the previous two
versions, we need to keep 3 builds.

To make generation of incremental mars easier, we use the version as the
directory name (instead of just the date).

We also define the tbb_version option, which is used to set the
TORBROWSER_NIGHTLY_VERSION environment variable.