[release planning] GRASS GIS 8: create new release branch

Hi devs,

Let me suggest to create a new release branch in the next two weeks
It will help us to focus on the upcoming and even announced 8.0.0
release: several conferences are upcoming with presentations on GRASS
GIS 8 talks and demo sessions.
So it makes quite sense to have a tangible version available, at least
in a release branch :-)

Now, the question is, also to avoid excessive cherry-picking of fixes
and changes from master (to be called 8.1 then), which of the open PR
may be merged soon?

See e.g. here:
https://github.com/OSGeo/grass/milestone/4

See also (please add notes!):
Draft release notes: https://trac.osgeo.org/grass/wiki/Grass8/NewFeatures80

Thanks,
Markus