Add WSGI script and fix errors with pip install

[ajkyffin]

Description

• Adds wsgi.py for running with Apache mod_wsgi
• Adds requests as a run-time dependency
• Moves config.json(.example) into the datagateway_api package (previously hard-coded as being in the directory underneath the package directory which is the python site-packages directory when installed)

Testing Instructions

Mostly just need to check that I haven't broken anything.

• Review code
• Check GitHub Actions build
• Check that moving config.json hasn't broken anything
• Test installation into a virtualenv using pip
• (optional) Test running with mod_wsgi

[codecov]

Codecov Report

Merging #207 (1572c0c) into master (856c31e) will decrease coverage by 0.15%.
The diff coverage is 20.00%.

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
- Coverage   87.10%   86.94%   -0.16%     
==========================================
  Files          27       28       +1     
  Lines        2225     2229       +4     
  Branches      193      193              
==========================================
  Hits         1938     1938              
- Misses        246      250       +4     
  Partials       41       41              

Impacted Files	Coverage Δ
datagateway_api/wsgi.py	0.00% <0.00%> (ø)
datagateway_api/common/config.py	97.46% <100.00%> (ø)

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