Migrate from Bintray to Sonatype

[karenfeng]

What changes are proposed in this pull request?

As Bintray is being sunsetted shortly, we need a new staging repository. This PR migrates us from sbt-bintray to sbt-sonatype. As Sonatype does not allow downloads from their remote staging repo, we use the local staging repo for our staging tests.

Of note to release conductors:

• We now use PGP signing. Use gpg --full-generate-key to generate your personal key, and set export GPG_TTY=$(tty).
• You now need Sonatype account creds in $HOME/sbt/1.0/sonatype.sbt, on which you should run chmod 777. Contact me to get access.
• To avoid accidental promotions, we will perform manual promotions to Maven Central via the Nexus Repository Manager.

How is this patch tested?

• [ ] Unit tests
• [ ] Integration tests
• [x] Manual tests

Pushed a staging release and verified that staging tests pass.

[codecov[bot]]

Codecov Report

Merging #367 (9d43e8f) into master (b5fae35) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #367   +/-   ##
=======================================
  Coverage   93.69%   93.69%           
=======================================
  Files          95       95           
  Lines        4823     4823           
  Branches      473      473           
=======================================
  Hits         4519     4519           
  Misses        304      304           

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