merge geysigning changes here

this is a meta-issue to track our sister project, geysigning. over there, i have opened an issue for them to consider merging geysigning with monkeysign, so this is our side of the issue.

i have already opened #6 (key exchange) and #7 (closed) (xdg-email) to add extra functionalities to monkeysign. the two most important bits that are missing there is revoked and expired keys management. this is tracked in the Debian BTS, see:

#766129 (monkeysign should not use revoked keys to make signatures), now in #37 (closed)
#736548 (monkeysign: Reports "key is expired, cannot sign" on non-expired key), now in #36 (closed)
#723763 (monkeysign should not sign revoked uids), now in #33 (closed)

i'm waiting for feedback from thobias to see if there's anything I missed there.

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