[#906] Added configuration fields for more dynamic texts

[Bartvaderkin]

Also renamed some fields I added earlier (from _subtitle to _intro) and changed them to be TextField's to be more consistent with other fields with similar role/position.

[codecov-commenter]

Codecov Report

Merging #330 (c58e012) into develop (6f11dc2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #330   +/-   ##
========================================
  Coverage    96.45%   96.46%           
========================================
  Files          420      425    +5     
  Lines        12732    12757   +25     
========================================
+ Hits         12281    12306   +25     
  Misses         451      451           

Impacted Files	Coverage Δ
src/open_inwoner/configurations/admin.py	100.00% <ø> (ø)
src/open_inwoner/utils/context_processors.py	90.90% <ø> (ø)
...en_inwoner/components/templatetags/product_tags.py	100.00% <100.00%> (ø)
...ons/migrations/0022_siteconfiguration_more_text.py	100.00% <100.00%> (ø)
...itle_siteconfiguration_home_questionnaire_intro.py	100.00% <100.00%> (ø)
...lter_siteconfiguration_home_questionnaire_intro.py	100.00% <100.00%> (ø)
...le_siteconfiguration_select_questionnaire_intro.py	100.00% <100.00%> (ø)
...er_siteconfiguration_select_questionnaire_intro.py	100.00% <100.00%> (ø)
src/open_inwoner/configurations/models.py	91.89% <100.00%> (+0.28%)	⬆️

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[alextreme]

Wat mij betreft akkoord en mag gemerged worden. Weet wel dat er in 914 een gerelateerde taak is om de eerder doorgevoerde velden ook optioneel te maken. Zou je meteen mee kunnen nemen of we houden het bij deze aanpassing, beide zijn prima imho.