Add Timezone Data to DB Backend

[MRichards99]

This PR will close #225

Description

This change helps to make the DB backend become more aligned with the ICAT backend by having timezones when displaying datetimes on the API.

It seems the DB backend doesn't actually use timezones anywhere - I tried setting the timezone flag as pointed out in the docs but that didn't have any impact. Instead, I've made the DB backend display the local timezone for the API. This seems to be the best compromise I could find to make the two backends aligned.

The DB backend now uses the DateHandler so will have the same format for datetimes as the ICAT backend.

I also noticed that the get session details endpoint doesn't use timezones on the ICAT backend. I fixed that on 9b2dc7d.

Testing Instructions

Check that both backends have display datetime in the same format (including the timezone part of the output)

• Review code
• Check GitHub Actions build
• Review changes to test coverage

Agile Board Tracking

Connect to #225

[codecov]

Codecov Report

Merging #232 (9b2dc7d) into master (0c68549) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #232      +/-   ##
==========================================
+ Coverage   89.45%   89.48%   +0.02%     
==========================================
  Files          31       31              
  Lines        2296     2301       +5     
  Branches      191      191              
==========================================
+ Hits         2054     2059       +5     
  Misses        211      211              
  Partials       31       31              

Impacted Files	Coverage Δ
datagateway_api/common/constants.py	100.00% <100.00%> (ø)
datagateway_api/common/database/models.py	96.43% <100.00%> (+0.01%)	⬆️
datagateway_api/common/icat/helpers.py	92.76% <100.00%> (+0.03%)	⬆️

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[sam-glendenning]

Works fine. Example output with a GET request to /sessions:

{
    "id": "e5525746-b235-11eb-bbcb-000c2906e44d",
    "expireDateTime": "2021-05-12 01:50:11-07:00",
    "username": "simple/root"
}