Add templates and stale rules for Github issues

These are directly inspired by those used by the Vienna RSS organization

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Report a problem with UniversalMailer
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Describe the bug**
+A clear and short description of what the bug is.
+
+**To Reproduce**
+Steps to reproduce the behaviour, if possible.
+
+**Screenshots**
+Add screenshots to help explain your problem, if possible.
+
+**Please complete the following information:**
+ - UniversalMailer version [e.g. 3.0.5]
+ - Mail.app version [e.g. Version 15.0 (3693.20.0.1.32)]
+ - OS version: [e.g. macOS 12.1]
+
+**Additional information:**
+In which version of UniversalMailer and Mail.app did the problem not occur, if applicable.

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for UniversalMailer
+title: ''
+labels: 'feature request :bulb:'
+assignees: ''
+
+---
+
+**Is your feature request related to a problem? Please describe.**
+A clear and short description of what the problem is.
+If there was a Github Discussion related to it, please provide a link.
+
+**Describe the solution you would like**
+A clear and short description of what you want to happen.
+
+**Describe alternatives you have considered**
+A clear and short description of any alternative solutions or features you have considered.
+
+**Additional context**
+Add any other context or screenshots about the feature request here.

.github/stale.yml

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+# Number of days of inactivity before an issue becomes stale
+daysUntilStale: 366
+
+# Number of days of inactivity before a stale issue is closed
+daysUntilClose: 30
+
+# Issues with these labels will never be considered stale
+exemptLabels:
+  - pinned
+  - security
+
+# Set to true to ignore issues in a project (defaults to false)
+exemptProjects: true
+
+# Set to true to ignore issues in a milestone (defaults to false)
+exemptMilestones: true
+
+# Label to use when marking an issue as stale
+staleLabel: "stale ⏳"
+
+# Comment to post when marking an issue as stale. Set to `false` to disable
+markComment: >
+  This issue has been automatically marked as stale because it has not had
+  recent activity. It will be closed if no further activity occurs. Thank you
+  for your contributions.
+
+# Comment to post when closing a stale issue. Set to `false` to disable
+closeComment: false
+
+only: issues