[#1978] Make uploadbutton and title show only when files are selected

[jiromaykin]

Issue: https://taiga.maykinmedia.nl/project/open-inwoner/task/1978

1. upload button must be invisible at first, + 'geselecteerde bestanden' must be invisible.
2. upload button becomes visible when file(s) are selected, + 'geselecteerde bestanden' becomes visible.
3. visible upload button becomes disabled if data-max-file-size is exceeded, but becomes enabled again when those selected files are removed with trash button.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ca8f7ab) 93.07% compared to head (575860d) 93.07%.
Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #943   +/-   ##
========================================
  Coverage    93.07%   93.07%           
========================================
  Files          828      828           
  Lines        28995    28995           
========================================
  Hits         26988    26988           
  Misses        2007     2007           

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