Refactor alerts script

[dracos]

This splits the big function that sends alerts into slightly smaller functions and therefore also tweaks the command line script so you can choose to send different types of alert rather than all at once.
It also fixes a small issue where "State changed to: open" is pointlessly shown in an update alert email.

[codecov]

Codecov Report

Merging #3615 (96d016d) into master (078fec6) will increase coverage by 0.01%.
The diff coverage is 95.91%.

❗ Current head 96d016d differs from pull request most recent head a6e7516. Consider uploading reports for the commit a6e7516 to get more accurate results

@@            Coverage Diff             @@
##           master    #3615      +/-   ##
==========================================
+ Coverage   82.61%   82.62%   +0.01%     
==========================================
  Files         346      346              
  Lines       23573    23581       +8     
  Branches     3588     3589       +1     
==========================================
+ Hits        19474    19484      +10     
+ Misses       2971     2969       -2     
  Partials     1128     1128              

Impacted Files	Coverage Δ
perllib/FixMyStreet/Script/Alerts.pm	91.53% <95.91%> (+0.57%)	⬆️
web/js/front.js	82.25% <0.00%> (-1.62%)	⬇️
web/cobrands/fixmystreet-uk-councils/alloy.js	90.09% <0.00%> (-1.00%)	⬇️
perllib/Open311.pm	90.09% <0.00%> (+1.59%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 078fec6...a6e7516. Read the comment docs.

[davea]

One little question, but 👍 overall.