Fix filtering by label

When clicking on a label on the AlertList or SilenceList view, the
changed Alert/Silence query is appended to the current url. This Postfix
should not start with a slash. If it does, the url is not appended but
replaced by the browser instead resulting in a `pushState` exception.

ui/app/src/Views/AlertList/Updates.elm

@@ -16,7 +16,7 @@ update : AlertListMsg -> Model -> Filter -> String -> String -> ( Model, Cmd Typ
 update msg ({ groupBar, filterBar } as model) filter apiUrl basePath =
     let
         alertsUrl =
-            basePath ++ "/#/alerts"
+            basePath ++ "#/alerts"
     in
         case msg of
             AlertsFetched listOfAlerts ->

ui/app/src/Views/SilenceList/Updates.elm

@@ -31,7 +31,7 @@ update msg model filter basePath apiUrl =
         MsgForFilterBar msg ->
             let
                 ( filterBar, cmd ) =
-                    FilterBar.update (basePath ++ "/#/silences") filter msg model.filterBar
+                    FilterBar.update (basePath ++ "#/silences") filter msg model.filterBar
             in
                 ( { model | filterBar = filterBar }, Cmd.map MsgForFilterBar cmd )