packaging: Update release tarball name and version scheme.

* extras/packaging/gnu-linux/scripts/release-version.sh: Add script
for deriving (reproducibly) a release version.  The format will follow
a 'YYYYMMDD.X' scheme, where 'YYYYMMDD' is the date of the last commit
on the current branch, and 'X' the number of releases tagged that day,
starting from zero.  Example: the first stable release with a newest
commit date of 2023-01-02 would have a version of '20230102.0', the
subsequent one '20230102.1', and so on.  This is derived by counting
the number of git tags with the prefix 'TYPE/YYYYMMDD', where 'TYPE'
is the release type: either 'stable', 'beta', or 'nightly'.
* extras/packaging/gnu-linux/Jenkinsfile: Call the above new script to
derive the release version to use for tagging.
* extras/packaging/gnu-linux/Makefile: Switch from the previous
'jami_YYYYMMDD.hhmm.COMMIT.tar.gz' tarball filename format to
'jami-YYYYMMDD.X.tar.gz'.  The new format is shorter, omits the
unnecessary and confusing commit id, and uses dash instead of
underscore.  Also, it contains everything in a top-level directory
with the same name as the tarball (without the .tar.gz extensions),
so that 'jami-YYYYMMDD.X.tar.gz' nicely extracts to 'jami-YYYYMMDD.X'.
With these changes, our release tarballs will follow more closely the
related instructions from the GNU maintainers manual:
https://www.gnu.org/prep/maintain/html_node/Distribution-tar-Files.html
Lastly, this change inadvertently 'fixes' a recent issue where the
'extras/ci/client-qt-gnulinux' subdirectory and its contents would be
moved to top-level in the tarball due to one of the tarball transforms
matching the 'client-qt' in the directory's name, replacing the
'extras/ci/client-qt' prefix with 'client-qt', thereby resulting in
'client-qt-gnulinux' being moved to the tarball's top-level.
* .gitignore: Ignore several packaging-related directories and files.
* extras/packaging/gnu-linux/rules/rpm/jami-daemon.spec:
* extras/packaging/gnu-linux/rules/rpm/jami-libclient.spec:
* extras/packaging/gnu-linux/rules/rpm/jami-qt.spec:
* extras/packaging/gnu-linux/rules/rpm/jami.spec: Update the expected
release tarball filename pattern in the 'Source' field.  Also replace
references to previous top-level directory name 'client-qt' with the
new one 'jami-%{version}' where needed.
* extras/packaging/gnu-linux/rules/rpm/jami-libqt.spec: Update the
'Source' field to follow new tarball name consistent with the main
Jami release tarball.
* extras/packaging/gnu-linux/scripts/build-package-rpm.sh: Update the
tarball top-level directory pattern for the new format, now passing
the --wildcards option to GNU Tar.  Also use a name for the jami-libqt
source tarball that is more consistent with the main Jami release
tarball.
* extras/packaging/gnu-linux/scripts/build-package-snap.sh: Update the
top-level directory pattern for the new format.
* extras/packaging/gnu-linux/tarballs.manifest: Drop file accidentally
checked into git.

Change-Id: I2695f560c0bc5e61fb99dc0244f8fbb8e7026a62