[GRASS-PSC] git migration

[Nikos Alexandris]

Hi all, a quick voice as non-core dev,

1. there should be one official GRASS GIS Add-Ons repository

2. there should be a set of minimal requirements to accept requests to
include add-ons in the official repository

3. `g.extension` deserves some love and improvements and it should issue
a Warning like "Note, you are installing an add-on from an unofficial
source" when using a repository other than the official one

4. there should be some minimal testing for an add-on, i.e.: does it
build after a(ny) commit?, does it work after a(ny) commit? and ideally
a set of unit tests too.

5. besides git, neither git-hub, nor git-lab should be ever considered
permanent

That said, import/export operations for repositories are by now quite
good and I guess they will improve further

Cheers, Nikos