Validate slack address in alert addresses

[johannaengland]

Just one of the many incoming alert engine tasks sparked by #1787.

[codecov]

Codecov Report

Merging #2657 (4afe04c) into 5.6.x (83095e7) will increase coverage by 0.08%.
The diff coverage is 100.00%.

❗ Current head 4afe04c differs from pull request most recent head 9aac97f. Consider uploading reports for the commit 9aac97f to get more accurate results

@@            Coverage Diff             @@
##            5.6.x    #2657      +/-   ##
==========================================
+ Coverage   54.04%   54.13%   +0.08%     
==========================================
  Files         558      558              
  Lines       40630    40641      +11     
==========================================
+ Hits        21958    22000      +42     
+ Misses      18672    18641      -31     

Files Changed	Coverage Δ
...on/nav/alertengine/dispatchers/slack_dispatcher.py	51.06% <100.00%> (+11.06%)	⬆️
python/nav/web/alertprofiles/forms.py	55.78% <100.00%> (+5.78%)	⬆️

... and 1 file with indirect coverage changes

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[github-actions]

Test results

     12 files       12 suites   11m 14s ⏱️
3 236 tests 3 140 ✔️   96 💤 0 ❌
9 183 runs  8 895 ✔️ 288 💤 0 ❌

Results for commit 9aac97f.

♻️ This comment has been updated with latest results.

[hmpf]

Nitpick, doc-improvement,otherwise ok.

[lunkwill42]

Looks good to me, agree with Hanne's nitpick :)

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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