This commit adds 'highlighted-element' class to <b> tags in
the following compose banners:
* jump_to_sent_message_conversation_banner
* stream_does_not_exist_error
* unknown_zoom_user_error
* channel wildcard_mention_not_allowed_error
* topic wildcard_mention_not_allowed_error
'highlighted-element' class sets the 'font-weight: 600',
replacing the default of 700.