TB-397 remove jembi superset dependency

[arran-standish]

Currently there exists this repo: https://github.com/jembi/superset which is only used to install a few python dependencies. However, this introduces the overhead of having to keep this custom docker image of superset in sync with apache's superset image. It can also cause some confusion when the latest tag on jembi/superset:latest does not correspond to apache/superset:latest.

This PR simply installs said python packages as part of the process of starting the superset image, and as such lets us use apache/superset image instead, while retaining the custom python packages we need.
Also included in this PR:

• Introduce postgresql as the database for superset instead of the default sqlite as per the superset recommendations (https://superset.apache.org/docs/installation/configuring-superset/#using-a-production-metastore)
• Parameterize the superset FEATURE_FLAGS to more easily allow updates to them and also removes the need to override the package to add/remove flags that platform sets.
• Add documentation detailing how to go about upgrading + rolling back a superset version upgrade
• Update ./build-image so that it will use the tag in config.yaml if no tag was passed in but there is a tag present in config.yaml

[MatthewErispe]

@arran-standish LGTM

stale

[rcrichton]

Looks good, just a small comment.

[rcrichton]

LGTM