Fix/update time timezone

[swissspidy]

Description Fixes #135 by using DATE_FORMAT to save the last updated time with timezone information. Also makes sure the getters always return DateTime objects, which makes handling this data easier anyway.

How has this been tested? Updated unit tests accordingly.

Types of changes Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] My code is tested.
• [x] My code follows the WordPress code style.
• [x] My code has proper inline documentation.

[codecov-io]

Codecov Report

Merging #136 into master will decrease coverage by 0.04%. The diff coverage is 92.3%.

@@             Coverage Diff              @@
##             master     #136      +/-   ##
============================================
- Coverage     79.63%   79.58%   -0.05%     
- Complexity      362      363       +1     
============================================
  Files            24       24              
  Lines           869      872       +3     
============================================
+ Hits            692      694       +2     
- Misses          177      178       +1

Impacted Files	Coverage Δ	Complexity Δ
inc/TranslationApiRoute.php	100% <100%> (ø)	5 <0> (ø)	:arrow_down:
inc/Project.php	100% <100%> (ø)	52 <4> (ø)	:arrow_down:
inc/Updater.php	93.93% <100%> (-0.1%)	26 <0> (ø)
inc/ZipProvider.php	98.68% <85.71%> (-1.32%)	24 <3> (+1)

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