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

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

[codecov[bot]]

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

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