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

config.example.py

@@ -33,3 +33,11 @@ SECRET_KEY = 'change-me'
 
 # Git repository base URL.
 URL = 'https://github.com/bui/taiko-web/'
+
+# Google Drive API.
+GOOGLE_CREDENTIALS = {
+    'gdrive_enabled': False,
+    'api_key': '',
+    'oauth_client_id': '',
+    'project_number': ''
+}