Content in Atom feed isn't necessarily XHTML

[snarfed]

@aciccarello reports that Inoreader is failing to ingest our Atom:

Here's validation output: https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Ffed.brid.gy%2Fuser%2Fciccarello.me%2Ffeed%3Fformat%3Datom . I expect it's_probably XML parsing error: :778:629: mismatched tag, due to us injecting HTML content from Mastodon directly into entries and claiming it's XHTML.

[snarfed]

Switched content from XHTML to HTML in CDATA. @aciccarello let me know if this made Inoreader happy!

[aciccarello]

I don't see any issues! Thanks for the fix. I'll probably check again tomorrow to make sure.