Add a powered by uMap footer

Fix #1304

umap/static/umap/js/umap.controls.js

@@ -1130,14 +1130,10 @@ L.U.Map.include({
     toggleCaveat()
     const download = L.DomUtil.create('a', 'button', container)
     download.textContent = L._('Download data')
-    L.DomEvent.on(
-      download,
-      'click',
-      () => {
+    L.DomEvent.on(download, 'click', () => {
       if (typeInput.value === 'umap') this.fullDownload()
       else this.download(typeInput.value)
-      }
-    )
+    })
     this.ui.openPanel({ data: { html: container } })
   },
 
@@ -1263,6 +1259,15 @@ L.U.AttributionControl = L.Control.Attribution.extend({
       const home = L.DomUtil.add('a', '', this._container, ` — ${L._('Home')}`)
       home.href = '/'
     }
+    if (this._map.options.captionMenus) {
+      const poweredBy = L.DomUtil.add(
+        'a',
+        '',
+        this._container,
+        ` — ${L._('Powered by uMap')}`
+      )
+      poweredBy.href = 'https://github.com/umap-project/umap/'
+    }
   },
 })