It's a number whichb must be ever increasing with each build submitted to the
store.

Automate its value by using the number of seconds since 1st of January 2019.
That should be enough for ~680 years.

This marks our switch to CalVer: http://calver.org/

Major: year
Minor: release number
Patch: build (in case we need to retry)

Set them to the next release versions. In additon, the buildNumber variable will
be used to match the requirements of versionCode:
https://developer.android.com/studio/publish/versioning

that is, a monotonically increasing number, independent of the app / sdk
version.

The variables in gradle.properties can be overridden on command line like: -Pversion=1.

As a step toward merging jitsi-meet-react with jitsi-meet to share as
much source code as possible between mobile and Web, merge the part of
jitsi-meet-react's source tree which supports mobile inside the
jitsi-meet source tree and leave jitsi-meet-react's Web support in the
source code revision history but don't have it in master anymore because
it's different from jitsi-meet's Web support. In other words, the two
projects are mechanically merged at the file level and don't really
share source code between mobile and Web.

As an intermediate step on the path to merging jitsi-meet and
jitsi-meet-react, import the whole source code of jitsi-meet-react as it
stands at
2f23d98424
i.e. the lastest master at the time of this import. No modifications are
applied to the imported source code in order to preserve a complete
snapshot of it in the repository of jitsi-meet and, thus, facilitate
comparison later on. Consequently, the source code of jitsi-meet and/or
jitsi-meet-react may not work. For example, jitsi-meet's jshint may be
unable to parse jitsi-meet-react's source code.