Migration to new jenkins system

[dylex]

Migration to jenkins-new, significant cleanup of Jenkinsfile, mostly maintains the same behaviors and tests.

• Fake downstream_ branches replaced with downstream parameter and clearer branch targets.
• Docker imaged replaced by local Dockerfile, which is just an optimized version of the old one, with some things removed (and downgraded back to clang++-6.0, since the image was based on an older Dockerfile)
• recordIssues is removed in favor of more -Werror arguments, though this could be reconsidered if there are other important features.

Remaining possible work:

• Add M1 mac builds
• Review and cleanup email result targets, including stan-buildbot list