search: fix search to limit dashboards better

Change the query to first select the dashboards, apply the limit then
join with tags. Means the limit will apply to the number of dashboards
returned in the search. The disadvantage is that the query will return
more rows than the limit, no. of dashboards x no. of tags. So hard limit
set to 5000 for all rows.

pkg/services/sqlstore/dashboard_test.go

@@ -3,6 +3,7 @@ package sqlstore
 import (
 	"testing"
 
+	"github.com/go-xorm/xorm"
 	. "github.com/smartystreets/goconvey/convey"
 
 	"github.com/grafana/grafana/pkg/components/simplejson"
@@ -12,9 +13,10 @@ import (
 )
 
 func TestDashboardDataAccess(t *testing.T) {
+	var x *xorm.Engine
 
 	Convey("Testing DB", t, func() {
-		InitTestDB(t)
+		x = InitTestDB(t)
 
 		Convey("Given saved dashboard", func() {
 			savedFolder := insertTestDashboard("1 test dash folder", 1, 0, true, "prod", "webapp")