Upgrade to modelmeta >= 0.3.0

[rod-glover]

This PR adjusts dependencies to make PDP use modelmeta >= 0.3.0. This means it can run against the pcic_meta database migrated to revision 12f290b63791. These changes include:

• Upgrading modelmeta and pdp_util versions in requirements.txt
• Removing version specifications for modelmeta and pdp_util from setup.py. This was causing build problems and is unnecessary.

Demonstration

In addition to the changes needed for the modelmeta upgrade, there are also the following:

• Fix broken Python CI build. Some changes in the underlying support for our existing build broke the tests. The self-hosted base image has been adjusted to ubuntu-18.04, and the build steps changed also to compensate the loss of support. Notes:
  • The base image for the CI is the same as the base image for our production and test Docker images, making them all very similar and consistent.
  • Function get_commitish() started raising a git exception in the Python CI build. Online advice on fixing this did not work. Instead, we have a fallback to the value "unknown" if this error occurs. It seems only to occur in the CI build.
• Fix broken production and test Dockerfiles, same reason.
• Fix broken Metadata hyperlink in portals. This fix should have been in a separate PR, but ... The fix itself is 4 characters; the tests to catch this type of error are more bulky.
• Apply Black to setup.py, which was getting messy.
• And, oops, a PR for Snyk accidentally merged into this branch. Oh well.

[rod-glover]

Snyk is flagging several unfixable vulns, so I am going to merge anyway and deal with the vulns later.