Auto-builder for nightly

[dbkr]

Some tools (or more likely, a tool) that:

* [ ] Watch for new tags and build the release tarball from them #12706
* [ ] Watch for new release tarballs and build the release electron apps #12707

• Watch for it being Nightly Time and build a nightly electron app

and probably also
* [ ] Watch the develop branch and build the develop tarball#12708

Build tool platform support:

• MacOS
• Linux
• 64 bit Windows
  * [ ] 32 bit Windows (pending Architecture read from node-gyp incorrectly neon-bindings/neon#490) not critical for nightlies

Other remaining bits:

• Sort out version mess on nightlies (just rename the files?)
  * [ ] Replace logo</del< Different logo for Riot Nightly #12709
• Gather assets and upload to server
• Override update URL
• Make a debian repo with both the deb we just built and the latest deb for the other channel
• Make a signing script for windows that passes the token password to signtool
• Figure out if we can access the keychain while screen is locked, move to files if not
• Give it a bot account & room
• Different debian package control file for nightly(!)

Split out of #11227

[jryans]

(Let's track for now as part of cross-signing since it's active work in support of that.)

[dbkr]

Have narrowed the scope of this to just nightlies and split the rest out to other bugs, so this is now done!