Electron nightlies

[turt2live]

• Set up a buildkite job to run at 9am UK time Mon-Fri (or every day if that's difficult). Time subject to opinions.
• Job generates unsigned tarballs and electron builds, similar to Automatically start a release buildkite job for release branches #11585
• Job automatically publishes to packages.riot.im (or whatever host we end up deciding upon).
• Use a different icon in the builds
• Use a different data directory for these builds.

We are using a separate binary for ease of deployment and for some semblance of safety.

Depends upon #11582 and #11585

[turt2live]

Fixes #8299

[aaronraimist]

Different icon is #12709

[jryans]

@turt2live Unsure if this meta issue still has value, what do you think? We've done some of the things, but in different ways, and other parts are tracked separately in different issues.

[turt2live]

Yea, let's use labels.