[#2946] Fix login for eHerkenning user with single vestiging

[pi-sigma]

The KVK branch selection view should be displayed, and the vestigingsnummer of the company should be stored in the session, for companies that have only a single vestiging (i.e. a hoofdvestiging but no nevenvestiging).

We currently skip the KVK branch selection view for companies with a single vestiging. The reason for doing this was a problem with endless redirects caused by our middleware (see https://taiga.maykinmedia.nl/project/open-inwoner/task/2000).

The issue now is that if a client logs in with a KVK nummer that has only one vestiging, the vestigingsnummer is not stored in the session. Consequently, any calls to APIS that make use of the KVK + vestigingsnummer will give the wrong results for this KVK number.

Reproducible with KVK number 90002490, which has only a hoofdvestiging.

Taiga: https://taiga.maykinmedia.nl/project/open-inwoner/task/2946

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.13%. Comparing base (0564935) to head (7cfe067).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1535   +/-   ##
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  Coverage    94.13%   94.13%           
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  Files         1064     1064           
  Lines        39271    39276    +5     
========================================
+ Hits         36967    36972    +5     
  Misses        2304     2304           

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[swrichards]

One note about the copy:

Log in namens alle vestigingen

Is a bit misleading: what you're actually doing is "Log in namens rechtspersoon" which specifically excludes all vestigingen, including the hoofdvestiging.

[swrichards]

LGTM, just some contextual things, plus my comment about a change to the form copy, to avoid confusion about rechtspersoon versus "all branches".

I double checked this locally, and it looks like it's sending the correct values to both Mijn Aanvragen and Mijn Vragen, depending on the selection (either kvk or vestigingsnummer for Mijn Vragen, depending on selection, and either kvk or kvk + vestiging for Mijn Aanvragen).