[#1979] Change date captions for cases list as design

[jiromaykin]

issue https://taiga.maykinmedia.nl/project/open-inwoner/task/1979
Dates in case list were not put in the positions they needed to be, so now put conforming to design (closely aligned to the Case title)
And: also improved some of the Card paddings according to design + changed the name of the gray variant to a "contextual" name that is more commonly used by designers and with design-tokens ('900' instead of '90').

Link to design: https://www.figma.com/file/iKGhWhstaLIlFSaND2q7cE/OIP---Designs-(new)?type=design&node-id=1%3A273&mode=design&t=PfzsswiRMAp5biaL-1

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (bcfa399) 94.75% compared to head (9e4b325) 94.75%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #959   +/-   ##
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  Coverage    94.75%   94.75%           
========================================
  Files          869      869           
  Lines        30288    30288           
========================================
  Hits         28698    28698           
  Misses        1590     1590           

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[pi-sigma]

Now that the date has been moved closer to the title, the distance between two lines of a multi-line title is bigger than the distance between title and date. Should we change this?

[jiromaykin]

Now that the date has been moved closer to the title, the distance between two lines of a multi-line title is bigger...

I have changed the line-height and changed the default text for open submissions button.