[bugfix] Use better plaintext representation of status for filtering

[tsmethurst]

Description

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This pull request updates our filtering logic to not use our SanitizeToPlaintext function for reducing status HTML content to plaintext, but instead use https://github.com/k3a/html2text, which doesn't cause weird line concatenation, and can competently extract links, mentions, and hashtags properly from the text.

To avoid re-parsing a status from HTML every time we want to filter it, a TTLCache has been added to the converter which stores the parsed-to-text version of statuses.

Also some minor fixes to our filter regexes, to include whitespace and start/end line in our whole word match.

closes https://github.com/superseriousbusiness/gotosocial/issues/3298 closes https://github.com/superseriousbusiness/gotosocial/issues/3128

Checklist

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• [x] I/we have read the GoToSocial contribution guidelines.
• [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• [x] I/we have not leveraged AI to create the proposed changes.
• [x] I/we have performed a self-review of added code.
• [x] I/we have written code that is legible and maintainable by others.
• [x] I/we have commented the added code, particularly in hard-to-understand areas.
• [x] I/we have made any necessary changes to documentation.
• [x] I/we have added tests that cover new code.
• [x] I/we have run tests and they pass locally with the changes.
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