bionic branch reshuffle

[Harald Sitter]

now that bionic is the new current series I've reshuffled our git branches

TLDR: If you want to make a change in xenial you need to use the new
_xenial branches! If you want to make a change in both xenial and
bionic you need to make it in the _xenial branches and the regular
Neon/whatever branches.

all repos that previously had a Neon/whatever branch now have this
branch stored as Neon/whatever_xenial. This acts as a (historic)
backup and is used to do the xenial builds until those go away.
all repos which previously had a Neon/whatever_bionic branch now had
this branch moved over the Neon/whatever branch. this means that some
repos may have a history break in those branches as not all repos
(notably: neon/seeds and forks/live-build) could be merged at all. So,
in case of rebase problems you'll likely need to rest to
origin/Neon/whatever (https://phabricator.kde.org/T9836 contains a
probably exhaustive list of repos where this *could* happen).

all Neon/whatever_bionic branches are gone now.

all jobs should have been updated and use the correct branches for the
series they build.

also, as a related aside: xenial builds no longer trigger on git
pushes but only build daily. this is in preparation for discontinuing
the builds entirely

HS