replace mapbox imagery with dedicated api tokens

scripts/update_imagery.js

@@ -4,8 +4,11 @@ let sources = require('editor-layer-index/imagery.json');
 const prettyStringify = require('json-stringify-pretty-compact');
 
 if (fs.existsSync('./data/manual_imagery.json')) {
+  const manualImagery = JSON.parse(fs.readFileSync('./data/manual_imagery.json'));
   // we can include additional imagery sources that aren't in the index
-  sources = sources.concat(JSON.parse(fs.readFileSync('./data/manual_imagery.json')));
+  sources = sources
+    .filter(source => !manualImagery.find(manualSource => manualSource.id === source.id))
+    .concat(manualImagery);
 }
 
 let imagery = [];