Use Rails tag API to build RSS feed for spoilers and polls (#20163)

* Use Rails tag API to build RSS feed for spoilers and polls

While the previous method did not contain a bug or a potential issue,
the tag API can be very resilient against future problems and reduces the
amount of manual management of the escape status of the content.

I've added tests to ensure that the formatting is broken and still
escapes control characters correctly.

* this seems cleaner and passes

* Incorporate feedback by moving the br to its own line and using the tag helper over the string constant for the br tag itself

* whoops, tag helper doesn't use a self-closing tag

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