Make mysqlclient a production-only requirement

[kevinrue]

Description

Following instructions in the README as a potential first time contributor, I ran into issue #574

This PR implements the solution discussed in the issue, namely:

• remove mysqlclient from core requirements
• create a new set of 'production' requirements that includes only mysqlclient
• ran pip-compile to regenerate core and dev requirements
• Because I haven't installed third-party MySQLclient yet, manully created and edited prod-requirements.txt to what it should look like. Once I confirm that the dev-requirements work, I'll try again pip-compile-ing the prod-requirements.txt file properly.

Also update the README file to add a one-time setup step running 'migrate' to initialise database tables.

Please include a summary of the change and which issue is fixed (if any). Please also
include relevant motivation and context. List any dependencies that are required for
this change.

Fixes #574

Type of change

• Documentation (non-breaking change that adds or improves the documentation)
• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Technical work (non-breaking, change which is work as part of a new feature)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass (eg. python -m pytest)
• The documentation builds and looks OK (eg. mkdocs serve)
• Pre-commit hooks run successfully (eg. pre-commit run --all-files)

Further checks

not applicable

• Code is commented, particularly in hard-to-understand areas
• Tests added or an issue has been opened to tackle that in the future. (Indicate issue here: # (issue))

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[AdrianDAlessandro]

Thanks!