Refactor backend styles / markup to use more NL DS components

[sergei-maertens]

Closes #3178

Depends on open-formulieren/design-tokens#42
Depends on open-formulieren/open-forms-sdk#592

---

How to test:

# build frontend/SDK with updated design tokens
cd /path/to/sdk
git switch refactor/3178-page-styles-to-nlds
git submodule update
yarn build:design-tokens
yarn build

# run backend with updated frontend build
cd /path/to/backend
npm i
npm run build
src/manage.py runserver

# check out a form via http://localhost:8000/some-form

---

Quite an extensive refactor with lots of things removed.

• Now uses utrecht-page and utrecht-page-content
• Now uses utrecht-page-header
• Now uses utrecht-page-footer
• Removed usage of openforms-layout

Due to some CSS cleanup the width on desktop has now gone from 1170px to 1200px - this is deliberate-ish to simplify the code and should not cause problems, but design tokens can of course be tweaked.

The changes should be backwards compatible - they're definitely intended to be, if you spot issues -> please report them.

TODO:

• Set up token data migration(s)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cd37b41) 95.97% compared to head (5027432) 95.97%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3575   +/-   ##
=======================================
  Coverage   95.97%   95.97%           
=======================================
  Files         680      680           
  Lines       21888    21890    +2     
  Branches     2532     2532           
=======================================
+ Hits        21006    21008    +2     
  Misses        611      611           
  Partials      271      271           

Files	Coverage Δ
src/openforms/config/models.py	97.27% <100.00%> (+0.03%)	⬆️

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