activity: add filtered count route

separated the search parts of ServerSearchReqDTO into
ServerFilterReqDTO, factored out part of GetActivities that made the
(filtering) query, which is then used in GetFilteredActivityCount with
db.Count.

router.go

@@ -285,6 +285,7 @@ func (app *appContext) loadRoutes(router *gin.Engine) {
 		api.POST(p+"/activity", app.GetActivities)
 		api.DELETE(p+"/activity/:id", app.DeleteActivity)
 		api.GET(p+"/activity/count", app.GetActivityCount)
+		api.POST(p+"/activity/count", app.GetFilteredActivityCount)
 
 		if userPageEnabled {
 			user.GET("/details", app.MyDetails)