Include username in email headers (#28981)

Include username in email headers (#28981)

Emails from Gitea comments do not contain the username of the commenter
anywhere, only their display name, so it is not possible to verify who
made a comment from the email itself:

	From: "Alice" <email@gitea>
	X-Gitea-Sender: Alice
	X-Gitea-Recipient: Bob
	X-GitHub-Sender: Alice
	X-GitHub-Recipient: Bob

	This comment looks like it's from @alice.

The X-Gitea/X-GitHub headers also use display names, which is not very
reliable for filtering, and inconsistent with GitHub's behavior:

	X-GitHub-Sender: lunny
	X-GitHub-Recipient: gwymor

This change includes both the display name and username in the From
header, and switches the other headers from display name to username:

	From: "Alice (@fakealice)" <email@gitea>
	X-Gitea-Sender: fakealice
	X-Gitea-Recipient: bob
	X-GitHub-Sender: fakealice
	X-GitHub-Recipient: bob

	This comment looks like it's from @alice.