Fix line-heights and convert scss calculations to calc()

bigbluebutton-html5/imports/ui/components/button/styles.scss

...omponents/chat/message-list/message-list-item/styles.scss

...ton-html5/imports/ui/components/dropdown/list/styles.scss

...tton-html5/imports/ui/components/modal/simple/styles.scss

...on-html5/imports/ui/stylesheets/variables/typography.scss