Create independent title string for 'Map' page

The "map" page didn't have a separate Transifex string for the page title. Instead the string 'label_map' was being used, which is also used elsewhere on the website. So no good idea to change that.
To make the page title independent from 'label_map' the title string is changed to 'map_title' (like on shops page, producers page etc.).

app/views/map/index.html.haml

@@ -1,5 +1,5 @@
 - content_for(:title) do
-  = t :label_map
+  = t :map_title
 
 - content_for :injection_data do
   = inject_available_countries