Show startcode at fints tan challenge

[Crasher508]

Bank import: After input online banking username & pin, a tan challenge start. Pycroft show a qr code, which we have to scan with our tan device. It response firstly with the startcode of this transaction. To check, if challenge started correctly, pycroft now can show the startcode too.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.89%. Comparing base (d941c3f) to head (be08759).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #771      +/-   ##
===========================================
+ Coverage    87.88%   87.89%   +0.01%     
===========================================
  Files          275      277       +2     
  Lines        16949    16989      +40     
===========================================
+ Hits         14895    14933      +38     
- Misses        2054     2056       +2     

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