Build release electron app with auto builder

[dbkr]

When the auto-builder finishes building a riot-web tarball (ie. making the github release) build the release electron app and upload it to packages in the same way as we do for nightly builds.

Blocked on neon-bindings/neon#491 for 32 bit support
Split out from #12452

[jryans]

The Windows 32-bit portion has been deferred to after release, tracked in #13175.

I'm taking a slightly different approach to this for the moment to save time: the auto-builder pulls the tarball from GitHub which was previously assembled by the build laptop.

[jryans]

Fixed by element-hq/element-builder#23