webapp/map: add OpenStreetMap attribution to Leaflet slippy map (#19)

* webapp/map: add OpenStreetMap attribution to Leaflet slippy map

Add attribution to the slippy map, formatted as shown in the VueJS
Leaflet docs: https://vue2-leaflet.netlify.app/quickstart/

The [ODbL](https://opendatacommons.org/licenses/odbl/1-0/) requires attribution
to be included alongside the map, stated in clause 4.3:
> You must include a notice associated with the Produced Work reasonably
> calculated to make any Person that uses, views, accesses, interacts with, or
> is otherwise exposed to the Produced Work aware that Content was obtained
> from the Database, Derivative Database, or the Database as part of a
> Collective Database, and that it is available under this License.

See also https://wiki.openstreetmap.org/wiki/Community_attribution_advice

Signed-off-by: Galen CC <galen8183@gmail.com>

* webapp/map: inline OpenStreetMap attribution

Signed-off-by: Galen CC <galen8183@gmail.com>

---------

Signed-off-by: Galen CC <galen8183@gmail.com>

webapp/src/views/Map.vue

@@ -15,7 +15,7 @@
       :use-global-leaflet="false"
       @update:bounds="updateBounds"
       @ready="mapLoaded"
-      :options="{ zoomControl: false, attributionControl: false }"
+      :options="{ zoomControl: false }"
     >
       <l-control position="topleft">
         <form @submit.prevent="onSearch">
@@ -47,6 +47,7 @@
         :url="mapTileUrl"
         layer-type="base"
         name="OpenStreetMap"
+        attribution="&copy; <a target=&quot;_blank&quot; href=&quot;http://osm.org/copyright&quot;>OpenStreetMap</a> contributors"
       />
 
       <l-control position="bottomright">