[SLWP] Kands collection page changes

[MorayMySoc]

Adds js for updating the amount of selected items and if price change is imminent.

Changes text for introduction box.

Fixes bug where total not being updated on page load.

Fixes final part of https://github.com/mysociety/societyworks/issues/3937
[skip changelog]

[codecov]

Codecov Report

Merging #4655 (9ace689) into kands-bulky-setup (3093845) will decrease coverage by 13.38%.
The diff coverage is n/a.

❗ Current head 9ace689 differs from pull request most recent head d6a75b6. Consider uploading reports for the commit d6a75b6 to get more accurate results

@@                  Coverage Diff                   @@
##           kands-bulky-setup    #4655       +/-   ##
======================================================
- Coverage              82.59%   69.21%   -13.38%     
======================================================
  Files                    378       42      -336     
  Lines                  28855     4951    -23904     
  Branches                4506        0     -4506     
======================================================
- Hits                   23832     3427    -20405     
+ Misses                  3678     1524     -2154     
+ Partials                1345        0     -1345     

see 336 files with indirect coverage changes

[dracos]

Suggesting using the same source of data in the JS, and bit of JS simplifying maybe :)