Fix truncated merge tags in HTML attribute when included in PDF setting Rich Text fields

[jakejackson1]

Description

Only field merge tags were being matched during rich text sanitizing. This adjustment now matches any merge tag with opening and closing {}, provided it is the only content included in a HTML attribute.

Testing instructions

1. Add <img src="{user:user_profile_picture}" /> to Footer HTML field
2. Save PDF settings
3. Verify the merge tag hasn't been truncated

Checklist:

• I've tested the code.
• My code is easy to read, follow, and understand
• My code follows the accessibility standards.
• My code has proper inline documentation / docblocks.

Additional Comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@                 Coverage Diff                 @@
##             hot-patch-6.11.2    #1562   +/-   ##
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  Hits                        ?     9887           
  Misses                      ?     3104           
  Partials                    ?        8           

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