Bugfix/improve db sessions

[louise-davies]

Description

These are the changes to improve the session handling that have been on preprod for months. Essentially, the session_manager object was flawed, and reading the session docs for SQLAlchemy (https://docs.sqlalchemy.org/en/13/orm/contextual.html), and it suggests using an "off-the-shelf" solution in Flask-SQLAlchemy if we're unfamiliar with session handling/threading. I tried it as a quick fix, and it seems a lot more stable, so it's easier to just import Flask-SQLAlchemy purely for its session handling than to try and fix the original solution.

Testing Instructions

• Review code
• Check GitHub Actions build
• Review changes to test coverage
• Verify that preprod (which this branch is deployed on) is not having regular 500 errors due to sessions.

[codecov]

Codecov Report

Merging #204 (7faee5c) into master (460b680) will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
- Coverage   87.25%   87.10%   -0.16%     
==========================================
  Files          28       27       -1     
  Lines        2221     2225       +4     
  Branches      192      193       +1     
==========================================
  Hits         1938     1938              
- Misses        242      246       +4     
  Partials       41       41              

Impacted Files	Coverage Δ
...way-api/datagateway_api/common/database/helpers.py	76.72% <0.00%> (-1.25%)	⬇️
...datagateway_api/common/database/session_manager.py
...gateway-api/datagateway_api/src/api_start_utils.py	86.36% <0.00%> (+1.94%)	⬆️

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[MRichards99]

I'm happy with these changes - I've tested them locally and on preprod and didn't experiences any issues on either.