Match mentions later because they are less specific

Match mentions later because they are less specific

Parts like `@example` can legitimately appears as part of a link that gets auto-detected.

This issue was discovered when an URL was pasted that happens to also match a user that is named `document`. The "offending" URL was: `https://developer.mozilla.org/de/docs/Web/CSS/@document`

The `@document` is recognized as part of a mention because the forward slash is a valid token that matches the boundary condition (`\b`) of the regex for mentions.

See https://community.woltlab.com/thread/292020-automatische-link-umwandlung-schl%C3%A4gt-fehlt/