[Merton] Add larger bin process flow.

[dracos]

Fixes https://github.com/mysociety/societyworks/issues/4594
[skip changelog]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.48%. Comparing base (a7366f4) to head (577af9a).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5251      +/-   ##
==========================================
+ Coverage   82.39%   82.48%   +0.08%     
==========================================
  Files         409      410       +1     
  Lines       32076    32250     +174     
  Branches     5119     5139      +20     
==========================================
+ Hits        26428    26600     +172     
- Misses       4127     4130       +3     
+ Partials     1521     1520       -1     

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[MorayMySoc]

This seems to work well for me - not sure if the email should adapt to say same as confirmation page is all that crossed my mind