Support adding GPG signing sub-key

[etanot]

Type : Feature request

Description

As @a1012112796 has mentioned in #14009 (comment) "Now gitea only allow add public gpg keys bounded with the user's email", it would be nice if Gitea also support adding GPG sub-key, just like mention in issue #14009.

[quhsi]

Currently, the email address cannot be kept private when using the PGP key feature. So this issue is more a bugfix than a feature.

[zeripath]

#14054 was an attempt at fixing this issue - however, due to the lack of real life testing of this feature I've had to move it to the 1.15 milestone. Hopefully we'll merge after the merge window for 1.14 is closed.

If you really want this feature in 1.14 you will need to download and test this PR to ensure that it does what you want.