[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 #3298
closes #3128

Checklist

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