BEF 1.3.1 May 2021 Launch of Update Site, and Tech Debt

This is almost entirely a devops and tech debt release for us. There are two user-visible changes:

• A small usability bug was partially fixed. Now all file types that exist in the root of each Bazel package will appear in the Project/Package Explorers. See #180 for details and limitations.
• The new BEF CI workflow produces a release build and publishes it to our new Eclipse update site, hosted on GitHub Pages. This means users no longer need to download the BEF archive to install it, just point Eclipse to our update site. (thanks @guw)

Additional internal changes, that are not visible to users:

• BEF is now built on the command line using the Maven Tycho plugins. This modernized our build process. Release builds are now cut from command line builds instead of exported from the Eclipse SDK. (thanks @guw)
• Now that we have a command line build, BEF also now has CI support. We have build workflows defined using GitHub Actions (thanks again @guw)
• Removed our internal dependency on Guava for the BEF code base, since we were barely using it. See #219