Add backend support for `<spoiler-text>`

Add backend support for `<spoiler-text>`

This produces a `<span>` with:

- A `property` attribute of `"tag:ns.1024.gdn,2022-11:spoiler_text"`.

- A `content` attribute of the original text contents of the
  `<spoiler-text>` element.

- Text contents which replace ASCII basic latin characters with their
  ROT13 equivalents and other alphabetic characters with REPLACEMENT
  CHARACTER. (The intent is that ROT13 gives a reasonable fallback for
  unsupporting servers for Latin‐script posts.)

`property` and `content` are RDFa attributes and don’t have any notable
security implications. They are disallowed on local posts because users
should just use `<spoiler-text>` directly instead.

Getting this to render nicely in the frontend is future work :) .