Add filter scope preview to planned maintenance form

[Simrayz]

Scope and purpose

Resolves #1765

This PR adds a preview of incidents that are covered by the currently selected filters in the Planned Maintenance form.
This should give a good indication of the extent of the maintenance task, to avoid accidentally generating too many notifications.

Screenshots

Contributor Checklist

Every pull request should have this checklist filled out, no matter how small it is.
More information about contributing to Argus can be found in the
Development docs.

• Added a changelog fragment for towncrier
• Added/amended tests for new/changed code
• Added/changed documentation, including updates to the user manual if feature flow or UI is considerably changed
• Linted/formatted the code with ruff and djLint, easiest by using pre-commit
• The first line of the commit message continues the sentence "If applied, this commit will ...", starts with a capital letter, does not end with punctuation and is 50 characters or less long. See our how-to
• If applicable: Created new issues if this PR does not fix the issue completely/there is further work to be done
• If this results in changes in the UI: Added screenshots of the before and after
• If this results in changes to the database model: Updated the ER diagram

[github-actions]

Test results

    6 files    894 suites   1m 58s ⏱️
  704 tests   703 ✅ 1 💤 0 ❌
4 224 runs  4 218 ✅ 6 💤 0 ❌

Results for commit 9e96035.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.68%. Comparing base (7f6da04) to head (9e96035).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1770      +/-   ##
==========================================
+ Coverage   82.57%   82.68%   +0.11%     
==========================================
  Files         137      137              
  Lines        6552     6574      +22     
==========================================
+ Hits         5410     5436      +26     
+ Misses       1142     1138       -4     

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud