enhance: allow custom watches to bypass on-mention email restriction (#39652) (#39837)

Backport #39652 by @Ubayed-Bin-Sufian

Users with "Only Email on Mention" can now still receive email for the
events they picked in a custom repository watch, covering issues, pull
requests and releases. The setting is renamed to "Only Email on Mention
and Custom Watches" to make this explicit, matching GitHub's
"Participating, @mentions and custom".

"All activity" watches don't bypass the setting, because they are also
created automatically by `AUTO_WATCH_NEW_REPOS` for repository creators
and team members. To keep picking every event distinct from "All
activity", the custom watch dialog now stores a new `WatchModeCustom`.
Existing custom watches keep working without a migration.

Addresses: https://github.com/openSUSE/openSUSE-git/issues/183
Ref: https://github.com/go-gitea/gitea/issues/35492
Co-authored-by: Ubayed Bin Sufian <ubayedbinsufian@gmail.com>
Co-authored-by: bircni <bircni@icloud.com>

routers/web/repo/watch.go

@@ -49,7 +49,7 @@ func ActionWatch(ctx *context.Context) {
 // ActionWatchOptions watches the repository with a custom selection of events
 func ActionWatchOptions(ctx *context.Context) {
 	opts := repo_model.WatchOptions{ // clearing every event is allowed, it leaves the participating state
-		Mode:              repo_model.WatchModeNormal,
+		Mode:              repo_model.WatchModeCustom,
 		WatchPullRequests: ctx.FormBool("pull_requests"),
 		WatchIssues:       ctx.FormBool("issues"),
 		WatchReleases:     ctx.FormBool("releases"),