[GitLab] Facilitate structured issue creation

We should make use of GitLab's features in using issue templates and proper labelling, at least for types of ~Bug and ~"Feature Request".

This involves:

• Creation of templates in a /.gitlab/issue_templates/ folder in the repository
• Templates should be clear and comprehensive enough that minimal effort is needed to understand the problem and aptly references technical information and examples if applicable
• Ensuring our users can create issues and label them accordingly
• In case labelling by an unprivileged user is impossible, we should at least have an automatic Triage flag so a staff member can label it instead
• Optionally determine whether the Service Desk feature of GitLab Premium (#96 (closed)) is a viable resource to associate with our forums