Fix styleling for `app/assets/stylesheets/framework/markdown_area.scss`

Problem

Fix the following errors for markdown_area.scss

app/assets/stylesheets/framework/markdown_area.scss
  53:1  ⚠  ".md-area" and ".div-dropzone                                         stylelint-gitlab/duplicate-selectors
           (/Users/filipalacerda/Documents/git/gitlab-development-kit/gitlab/
           app/assets/stylesheets/framework/markdown_area.scss -2:3)" have the
           same properties.
 180:3  ⚠  "small.aliases,   small.params" and "&.btn.btn-sm                     stylelint-gitlab/duplicate-selectors
           (/Users/filipalacerda/Documents/git/gitlab-development-kit/gitlab/
           app/assets/stylesheets/framework/markdown_area.scss -64:7)" have the
           same properties.
 225:9  ⚠  Expected nesting depth to be no more than 3                           max-nesting-depth
 225:9  ⚠  Expected ".atwho-view .atwho-view-ul li.cur small" to have no more    selector-max-compound-selectors
           than 3 compound selectors
 229:9  ⚠  Expected nesting depth to be no more than 3                           max-nesting-depth
 235:7  ⚠  Expected ".atwho-view .atwho-view-ul li div.avatar" to have no more   selector-max-compound-selectors
           than 3 compound selectors
 240:9  ⚠  Expected nesting depth to be no more than 3                           max-nesting-depth
 240:9  ⚠  Expected ".atwho-view .atwho-view-ul li div.avatar .center" to have   selector-max-compound-selectors
           no more than 3 compound selectors
 245:7  ⚠  Expected ".atwho-view .atwho-view-ul li strong" to have no more than  selector-max-compound-selectors
           3 compound selectors

Generic Guidelines:

1. If possible, replace the CSS class with a utility one, check common.scss and https://getbootstrap.com/docs/4.2/utilities/ for existing ones
2. In the MR, add screenshots of the affected areas before and after the changes
3. Label the MR with CSS cleanup