ImportSongs: Add Google Drive support

- Adds a new page for importing custom songs, where it is possible to pick a local folder (desktop only) or a Google Drive folder (desktop and Android)
  - This feature is disabled on iOS due to the lack of OGG audio support in the browser
- In order to not get rate limited, a TJA file is parsed for metadata only when the song is clicked in the song selection, rather than all at once at import time
- The instance maintainer will need to provide the API credentials in the config.py file to enable this feature
  - This requires a new project to be created at console.cloud.google.com
  - Drive API will have to be enabled
  - API and OAuth keys should be created
    - API key can be restricted to only have Google Drive and Google Picker APIs
    - OAuth Client ID should have Web Application type and JavaScript origins set
    - Editing the OAuth consent screen to have a name and icon is recommended
      - It is semi-required to submit the consent screen for verification as the permission to download all of the Drive files will be asked.
      - Note that the email of the maintainer is publicly visible on the consent screen
  - The project number can be found in the IAM & Admin settings page

app.py

@@ -119,7 +119,8 @@ def get_config():
         'assets_baseurl': config.ASSETS_BASEURL,
         'email': config.EMAIL,
         'accounts': config.ACCOUNTS,
-        'custom_js': config.CUSTOM_JS
+        'custom_js': config.CUSTOM_JS,
+        'google_credentials': config.GOOGLE_CREDENTIALS
     }
 
     if not config_out.get('songs_baseurl'):