Add QR Code Scanner

[Kore-rep]

Add a QR code scanner using Google ML Kit for Android.

The camera is opened via a neutral button on the add subscription button.
The code will be scanned and used to auto-populate the subscription dialog. The text in the codes should have the following format:
ntfy://https://server.com;topicname

Work to consider :

• Auto subscribing on scanning a valid URL.
• Better error messaging to help users debug.

If this work is merged in, I will also update the docs to include a section on QR codes

Built off the work on #67 and #87

[Kore-rep]

I now realize the Android App already has deep linking meaning QRs can be used straight from the default camera, feel free to close this if not required

[cyb3rko]

@Kore-rep Yeah, I think ntfy:// links are enough, we don't need additional camera overhead inside the app.

[wunter8]

I think there was some desire to extend the ntfy:// syntax to support configuring a server URL, topic, authentication, and a display name all in a single URL that could be embedded in a QR code. But I agree that we probably don't need to embed a QR code reader in the app