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

public/src/views/customsongs.html

@@ -0,0 +1,12 @@
+<div class="view-outer">
+	<div class="view">
+		<div class="view-title stroke-sub"></div>
+		<div class="view-content"></div>
+		<div class="left-buttons">
+			<div id="link-localfolder" class="taibtn stroke-sub link-btn"></div>
+			<div id="link-gdrivefolder" class="taibtn stroke-sub link-btn"></div>
+		</div>
+		<div class="view-end-button taibtn stroke-sub selected"></div>
+	</div>
+	<form><input id="browse" type="file" webkitdirectory multiple></form>
+</div>