Allow .age attachments to issues

[xmo-odoo]

Currently, github has neither private / hidden issues (#37) nor private / hidden attachments on public issues.

Age is a very simple and opinionated encryption tool, which as a bonus works extremely well with github: age -R <(curl -o- https://github.com/<username>.keys) < infile > outfile will encrypt "infile" using the user's github public keys (-R can be repeated for multiple recipients), allowing them to use their private github key to decrypt the file and giving nobody else access.

This means it works rather well as a way to upload attachments visible only to maintainers. Or would if age files were supported, as-is they have to be smuggled as the "cleartext" file (stripping out the .age extension), possibly confusing people.

[github-actions]

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[Levi-Lesches]

Duplicate of #37?

[xmo-odoo]

@Levi-Lesches not exactly, I'm using encrypted attachments as a work-around-ish for #37 (hence having linked to it in the description) but private attachments on public issues is useful regardless, and so is encrypted attachments in general.

[michellemerrill]

Closing per #2029 (comment) and #2035