Accessibility improvements to hint_style for emails

[lucascumsille]

Currently, the email's top(email_summary) and bottom(email_footer) parts don't necessarily pass the contrast test. The text colour uses body_text_color, and the background colour for the top and bottom sections is not set, so the hint text is above whatever the body_background_color is. Looking at some cobrands, I can see that the problem is not only with FMS. The solution on this commit gives hint_style a background that uses color_white as a variable, and because the text is using the body_text_color, it is less likely the contrast test will fail. The scenario where this could happen is when body_text_color is using a light colour, which, so far, I haven't seen a cobrand doing this.

Screen.Recording.2025-02-17.at.14.16.05.mov

Important:

• If the solution above is accepted then we need to update the email_top and email_bottom templates for: Hackney, Bathnes and Fixamingata
• Regarding getting rid of email_summary it does seem like it could be done. However looking at the email_summary content across cobrands it seems like it doesn't mimic the main title of the email 100% of the time. Reason why I didn't remove it.

I added an extra commit that corrects the extra space use by "This email was sent from a staging site."

Before

After

[skip changelog]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.42%. Comparing base (98764b8) to head (ba26387).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5372      +/-   ##
==========================================
- Coverage   82.43%   82.42%   -0.01%     
==========================================
  Files         416      416              
  Lines       32979    32979              
  Branches     5308     5308              
==========================================
- Hits        27186    27184       -2     
- Misses       4220     4221       +1     
- Partials     1573     1574       +1     

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