hotfix: make week handle timezone changes

[limwa]

This pull request makes the week util class handle timezone changes properly.

Dart's standard lib, when asked to add or subtract 7 days from a DateTime will, instead, add/subtract the equivalent of 7 days in seconds.

This results in wrong calculations when the week contains a timezone change. By copying the dates with isUtc: true, the calculations become correct, since UTC does not have timezone changes.

Review checklist

• Terms and conditions reflect the current change
• Contains enough appropriate tests
• If aimed at production, writes a new summary in whatsnew/whatsnew-pt-PT
• Properly adds an entry in changelog.md with the change
• If PR includes UI updates/additions, its description has screenshots
• Behavior is as expected
• Clean, well-structured code

[codecov]

Codecov Report

Merging #1217 (a14aa0c) into master (6f37a0d) will increase coverage by 1%.
Report is 2 commits behind head on master.
The diff coverage is 75%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1217   +/-   ##
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+ Coverage      17%     17%   +1%     
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  Files         229     229           
  Lines        6975    6979    +4     
======================================
+ Hits         1144    1155   +11     
+ Misses       5831    5824    -7