Adds signing workflow for releases

[Cawllec]

Goal

Adds a workflow that will:

• Download all assets from a given release (set automatically when triggered by a release)
• Create a signature for each asset using our platforms release GPG key
• Confirm that signatures are correct for the asset and key
• Upload the signatures to the release

Testing

A test run can be observed here, and the outcome of the test run can be observed here. Note that these did not use our platforms release key.

For full testing this will need to be merged and then manually run against a previous release.

[bugsnagbot]

Android notifier sizes

Format	Size impact of Bugsnag (kB)	Size impact of Bugsnag when Minified (kB)
APK	1862.82	1675.8
arm64_v8a	639.23	450.82
armeabi_v7a	573.7	385.29
x86	717.04	528.62
x86_64	684.28	495.86

Generated by :no_entry_sign: Danger