At mention and topics locally

The constructs from at mention and topics on web should also just work locally, and redirect to ourbigbook.com by default.

Once they work, document them with something like:

= `\x` `href` argument
{parent=`\x` sargument}

If the `href` argument starts with certain prefixes, magic links are generated:
* `@`: link to <OurBigBook.com> user profiles, e.g.:
  \OurBigBookExample[[
  I love \a[@cirosantilli], he is great!
  ]]
  links to: https://ourbigbook.com/cirosantilli

  TODO make it work without the `\a`, just: `@cirosantilli`.
* `#`: link to <OurBigBook.com> <OurBigBook Web topics>[topics]:
  \OurBigBookExample[[
  \a[#quantum-mechanics][Quantum mechanics] is very difficult to understand.
  ]]
  links to: https://ourbigbook.com/go/topic/quantum-mechanics

It is not perfectly elegant to use <> for this, especially locally, since it means linking to IDs that don't exist (on Web, @username is an actually regular ID on the DB. But #topic isn't). But perhaps just having the <> links to non-files is just the way to go.

At mention and topics locally

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