Initial Linux release

[alfred-landrum]

We want to provide installable Linux releases of Brim. Our CI already builds and executes Linux based tests (using Ubuntu & xfvb). We have user reports about desired distros in #586 (note the comments there about flatpak.org) and a few other in the public #brim slack channel.

We currently use the Electron update service for autoupdates:
https://www.electronjs.org/docs/tutorial/updates

which does not support autoupdates on Linux. It will be acceptable if we don't initially support auto-update on Linux, but the user shouldn't see any errors or warnings about auto-update not working.

We need to:

• decide on the packaging format(s) that we want to provide.
• add support for them in script/release as we do for other platforms
• add a CI trigger that will generate the release artifact & upload it as for other platforms
• add an icon and links to the Linux package(s) on the brimsecurity.com/download page.

[philrz]

Verified. I made .deb and .rpm release artifacts and installed them via double-click in the Files utilities of Ubuntu Desktop 18.04 and Fedora Workstation 31, respectively. Per the attached videos, in each I was able to drag a pcap into the app and open flows in Wireshark successfully.

Ubuntu.zip

Fedora.zip