Store event metadata as issue template

Closes #106 (closed)

This is a suggestion to collect all internal tasks for event orga in hifis/software-services-organization and an actionable announcement template here. Issue template, since we don't (need to) pay for the GitLab version that has custom file templates ;-)

See esp. https://gitlab.hzdr.de/hifis/software.hifis.net/issues/106#note_25350 and also this MRs companion over in https://gitlab.hzdr.de/hifis/software-services-organization/merge_requests/15