Add support for User SSH signing keys

[neersighted]

Introduce support for the new SSH signing key endpoints introduced into the REST API with the introduction of SSH commit verification.

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[gmlewis]

Thank you, @neersighted !
LGTM.

Awaiting second LGTM from any other contributor to this repo before merging.

[gmlewis]

@neersighted - please use regular push of commits to this PR and not force push, as it makes it really challenging for the reviewer to see what changes each time.

We always use squash and merge in this repo anyway so it doesn't matter how many commits it takes. Thank you.

[codecov]

Codecov Report

Merging #2482 (c3c2c61) into master (0c3cd12) will decrease coverage by 0.04%.
The diff coverage is 89.47%.

@@ Coverage Diff @@## master #2482 +/- ##
==========================================
- Coverage 98.03% 97.99% -0.05% 
==========================================
Files 122 123 +1 Lines 10705 10762 +57 ==========================================
+ Hits 10495 10546 +51 - Misses 144 148 +4 - Partials 66 68 +2 

Impacted Files	Coverage Δ
github/users_ssh_signing_keys.go	89.47% <89.47%> (ø)

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[neersighted]

Fair -- I'm usually used to a "clean history" workflow -- in this case go generate was missed.

[gmlewis]

Thank you, @valbeat !
Merging.