Add classes for /weakness command's labels

This adds in style for the new labels for the /weakness command, so that "Weaknesses:" is coloured red (the same as the red for .message-learn-cannotlearn for consistency reasons), "Resistances" is coloured blue, and "Immunities" is coloured grey. This reflects the changes to the command made in this pull request: https://github.com/Zarel/Pokemon-Showdown/pull/1116