collective sysadmin redmine user

Originally created by @groente on #16854 (Redmine)

we’d like to have a possibility of assigning tickets to the sysadmin team in general.
for this we’d need to:

- create a role user for our team on Redmine
- communicate about it to -summit@ (and in our doc?)
- update or deprecate https://redmine.tails.boum.org/code/projects/tails/issues?query_id=262 and https://redmine.tails.boum.org/code/projects/tails/issues?query_id=267