Add lint to CI and fix errors (#853)

Adds lint to CI and fixes errors.
I'm not sure I'm fully proud of all the solutions there.
But there's no lint issues and the lint is being checked as part of the
CI.

---------

Co-authored-by: Yuri Astrakhan <yuriastrakhan@gmail.com>

src/libs/metadata.ts

@@ -5,16 +5,16 @@ function loadJSON(url: string, defaultValue: any, cb: (...args: any[]) => void)
     mode: 'cors',
     credentials: "same-origin"
   })
-  .then(function(response) {
-    return response.json();
-  })
-  .then(function(body) {
-    cb(body)
-  })
-  .catch(function() {
-    console.warn('Can not metadata for ' + url)
-    cb(defaultValue)
-  })
+    .then(function(response) {
+      return response.json();
+    })
+    .then(function(body) {
+      cb(body)
+    })
+    .catch(function() {
+      console.warn('Can not metadata for ' + url)
+      cb(defaultValue)
+    })
 }
 
 export function downloadGlyphsMetadata(urlTemplate: string, cb: (...args: any[]) => void) {
@@ -22,14 +22,14 @@ export function downloadGlyphsMetadata(urlTemplate: string, cb: (...args: any[])
 
   // Special handling because Tileserver GL serves the fontstacks metadata differently
   // https://github.com/klokantech/tileserver-gl/pull/104#issuecomment-274444087
-  let urlObj = npmurl.parse(urlTemplate);
+  const urlObj = npmurl.parse(urlTemplate);
   const normPathPart = '/%7Bfontstack%7D/%7Brange%7D.pbf';
   if(urlObj.pathname === normPathPart) {
     urlObj.pathname = '/fontstacks.json';
   } else {
     urlObj.pathname = urlObj.pathname!.replace(normPathPart, '.json');
   }
-  let url = npmurl.format(urlObj);
+  const url = npmurl.format(urlObj);
 
   loadJSON(url, [], cb)
 }