Distribute Alpha extension on desktop

[devinreams]

• [x] package script to build xpi
• [x] added to internal sign-xpi Jenkins pipeline (via Test Pilot and CloudOps)
• [x] confirmed signed properly
• [x] confirmed updates distributed
• [x] process documented
• [x] write 0.1.0-alpha changelog #76 #176
• [x] publish 0.1.0-alpha tag and release on GitHub

[devinreams]

Thinking about how to package, sign, and distribute this over at #62

[sandysage]

Completed 10/16