Rename manifest.json to manifest.webmanifest (#1399)

* Rename manifest.json to manifest.webmanifest

browser consoles keep complaining that the manifest doesn't have the `.webmanifest` extension.

* FIx manifest.webmanifest references

Co-authored-by: Deluan <deluan@navidrome.org>

tests/fixtures/robots.txt

@@ -1,4 +1,4 @@
 User-agent: bingbot
-Disallow: /manifest.json
+Disallow: /manifest.webmanifest
 
 User-agent: *

ui/public/index.html

@@ -16,10 +16,10 @@
 
     <meta name="viewport" content="width=device-width, initial-scale=1" />
     <!--
-      manifest.json provides metadata used when your web app is installed on a
+      manifest.webmanifest provides metadata used when your web app is installed on a
       user's mobile device or desktop. See https://developers.google.com/web/fundamentals/web-app-manifest/
     -->
-    <link rel="manifest" href="%PUBLIC_URL%/manifest.json" />
+    <link rel="manifest" href="%PUBLIC_URL%/manifest.webmanifest" />
     <!--
       Notice the use of %PUBLIC_URL% in the tags above.
       It will be replaced with the URL of the `public` folder during the build.

ui/public/robots.txt

@@ -1,4 +1,4 @@
 User-agent: bingbot
-Disallow: /manifest.json
+Disallow: /manifest.webmanifest
 
 User-agent: *