Don't show mail-notification settings when ENABLE_NOTIFY_MAIL is false

[uriesk]

Description

I wasted time playing with mail-notification settings in the profile and adding / removing mail addresses.
Till i realized that there is a ENABLE_NOTIFY_MAIL setting, which is false per default.

I think the mail notification setting in the profile should not be available (grayed out), or there should be some
"Mail notifications are disabled by the Administration" or any other hint making it obvious that it is not available.

Screenshots

Gitea Version

1.17.0-rc1

Can you reproduce the bug on the Gitea demo site?

Yes

Operating System

No response

Browser Version

Brave 1.40.105 Chromium: 103.0.5060.53

[Francewhoa]

This is another example of a user struggling with notification at https://discourse.gitea.io/u/JVC-MSF

I believe that @uriesk's suggestions above would resolve this challenge