Create Progressive Web App (#4730)

* Create manifest and serviceworker

* Create templates and add AppSubUrl

* Add JSRenderer

* fix ctx type

* Add JSRenderer to static.go

* Complete adding {{AppSubUrl}}

* Add more fonts to urlsToCache

* Add 512px and 192px icons

* Hardcode font MD5

* Default theme doesn't have a specific CSS file

templates/pwa/manifest_json.tmpl

@@ -0,0 +1,31 @@
+{
+    "short_name": "Gitea",
+    "name": "Gitea - Git with a cup of tea",
+    "icons": [
+      {
+        "src": "{{AppSubUrl}}/img/gitea-lg.png",
+        "type": "image/png",
+        "sizes": "880x880"
+      },
+      {
+        "src": "{{AppSubUrl}}/img/gitea-sm.png",
+        "type": "image/png",
+        "sizes": "120x120"
+      },
+      {
+        "src": "{{AppSubUrl}}/img/gitea-512.png",
+        "type": "image/png",
+        "sizes": "512x512"
+      },
+      {
+        "src": "{{AppSubUrl}}/img/gitea-192.png",
+        "type": "image/png",
+        "sizes": "192x192"
+      }
+    ],
+    "start_url": "{{AppSubUrl}}/",
+    "scope": "{{AppSubUrl}}/",
+    "background_color": "#FAFAFA",
+    "display": "standalone",
+    "theme_color": "{{ThemeColorMetaTag}}"
+  }