Validate application builds.

As a security-conscious nextcloud user, I worry about the fact I can't validate the android application builds are not verifiable via cryptography (PGP or Android key hash)

For users acquiring the app builds via github or some other place, Nextcloud's download website should have the developer's signing key hash published for easy build validation. PGP is also an option, though it is harder for non technical users.

Example of SHA-256 certificate fingerprint (Appp bundle name followed by key hash):

```
com.nextcloud.client
FB:00:95:22:F6:5E:25:80:22:61:B6:7B:10:A4:5F:D7:0E:61:00:31:97:6F:40:B2:8A:64:9E:15:2D:ED:03:73
```