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