Changes re: client no longer running arbitrary server CSS/JavaScript

As of Zarel/Pokemon-Showdown-Client@5950a2dbbd, the client will no
longer run arbitrary JavaScript or `style` tags received from the
server. This commit makes various changes in response to this:

- Various chat commands now send `class` attributes rather than
  `style` attributes.

- The /redirect command is removed.

- The /banredirect command is removed.