Mirror sync interval specified as duration string (#1407)

* Sync interval specifed as duration string

* Changed mirror interval text

* make fmt

* Add MinInterval for mirror sync

* Use duration internally

* Changed min default to 10m

* make fmt

* Incorrect default

* Removed defaults in MustDuration()

* Add Mirror interval migration

* Default values corrected

* Use transaction during migration

* Change http 500 to page with error message

* Cleanup session.commit()

modules/auth/repo_form.go

@@ -88,7 +88,7 @@ type RepoSettingForm struct {
 	RepoName      string `binding:"Required;AlphaDashDot;MaxSize(100)"`
 	Description   string `binding:"MaxSize(255)"`
 	Website       string `binding:"Url;MaxSize(255)"`
-	Interval      int
+	Interval      string
 	MirrorAddress string
 	Private       bool
 	EnablePrune   bool