[Qgis-psc] Enable Probot-Stale on qgis/QGIS

[Matthias Kuhn]

Hi PSC,

after the recent cleanup of the pull request queue on github in the
final spurt towards QGIS 3.2, it was discussed to add a bot that closes
stale pull requests after some time to keep the pull request queue clean
and tidy in a sustainable manner.
It turns out, this exists and is quite easy to activate. I have setup
everything required in a pull request
(https://github.com/qgis/QGIS/pull/7311) and there seems to be a general
consensus among the developers that this is a desirable tool.
I don't have the power to install probot stale
https://probot.github.io/apps/stale/ on the repository myself. Is this
something that can be easily done by someone or do we need some sort of
formal decision?
Thanks in advance
Matthias