Snapshots: Add snapshot enable config (#61587)

* Add config to remove Snapshot functionality (frontend is hidden and validation in the backend)
* Add test cases
* Remove unused mock on the test
* Moving Snapshot config from globar variables to settings.Cfg
* Removing warnings on code

pkg/api/frontendsettings.go

@@ -209,6 +209,7 @@ func (hs *HTTPServer) getFrontendSettingsMap(c *models.ReqContext) (map[string]i
 		"samlEnabled":             hs.samlEnabled(),
 		"samlName":                hs.samlName(),
 		"tokenExpirationDayLimit": hs.Cfg.SATokenExpirationDayLimit,
+		"snapshotEnabled":         hs.Cfg.SnapshotEnabled,
 	}
 
 	if hs.ThumbService != nil {