Make mirror feature more configurable (#16957)

Rename`[repository]` `DISABLE_MIRRORS` to `[mirror]` `DISABLE_NEW_PULL`  and add `ENABLED` and `DISABLE_NEW_PUSH` with the below meanings:

- `ENABLED`: **true**: Enables the mirror functionality. Set to **false** to disable all mirrors.
- `DISABLE_NEW_PULL`: **false**: Disable the creation of **new** mirrors. Pre-existing mirrors remain valid.
- `DISABLE_NEW_PUSH`: **false**: Disable the creation of **new** push mirrors. Pre-existing mirrors remain valid.


Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: delvh <dev.lh@web.de>

routers/api/v1/repo/mirror.go

@@ -9,6 +9,7 @@ import (
 
 	"code.gitea.io/gitea/models"
 	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/setting"
 	mirror_service "code.gitea.io/gitea/services/mirror"
 )
 
@@ -42,6 +43,11 @@ func MirrorSync(ctx *context.APIContext) {
 		ctx.Error(http.StatusForbidden, "MirrorSync", "Must have write access")
 	}
 
+	if !setting.Mirror.Enabled {
+		ctx.Error(http.StatusBadRequest, "MirrorSync", "Mirror feature is disabled")
+		return
+	}
+
 	mirror_service.StartToMirror(repo.ID)
 
 	ctx.Status(http.StatusOK)