Plan an end of support of python 2 in grist

[hexaltation]

Describe the problem to be solved

For historical reasons python 2 is an engine used in https://github.com/gristlabs/grist-core .
Python 2 have reached its end of life on 1st of january 2020.
From a security point of view we think that a great idea to remove this piece which have not received
security patched since more than 5 years.

Describe the solution you would like

• Inform SaaS users, selfhosters on a clear date of end of life, so they can plan a migration in their instances/documents,
• Remove calls to python 2 in code
• Remove switch engine button in Document settings
• Remove python 2 from DockerFile

If needed, maybe we can start by keeping allowing it only under a feature flag.