Fix the style-lint errors and warnings for `app/assets/stylesheets/pages/wiki.scss`

Problem

Fix the following style-lint errors and warnings for app/assets/stylesheets/pages/wiki.scss

 10:3  ⚠  ".edit-wiki-page-slug-tip" and ".new-wiki-page-slug-tip (app/assets/stylesheets/pages/wiki.scss -2:3)" have the same         stylelint-gitlab/duplicate-selectors properties.
 25:3  ⚠  Expected a placeholder selector (e.g. %placeholder) to be used in @extend      

What to do

1. If possible, replace the CSS class with a utility one, check common.scss and https://getbootstrap.com/docs/4.3/utilities/ for existing ones
2. Double check if all the warning have been fixed by running yarn run stylelint-file app/assets/stylesheets/pages/wiki.scss
3. In the MR, add screenshots of the affected areas before and after the changes, check the first MR as a guideline
4. If there are a lot of changes, we recommend to break them into smaller MRs
5. Double check the usage of each changed class both in CE and EE
6. EE changes should either be backported into CE or moved into ee/ folder. You can read more about how to handle EE code here
7. Label the MR with ~CSS cleanup