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

Problem

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

 145:7  ⚠  "button" and "[v-cloak] (app/assets/stylesheets/pages/boards.scss -1:1)" have the same properties.                           stylelint-gitlab/duplicate-selectors
 171:5  ⚠  ".board-list-component,     .issue-count-badge" and "button (app/assets/stylesheets/pages/boards.scss -145:7)" have the      stylelint-gitlab/duplicate-selectors
           same properties.
 195:1  ⚠  ".board-header" and ".boards-app (app/assets/stylesheets/pages/boards.scss -69:1)" have the same properties.                 stylelint-gitlab/duplicate-selectors
 363:1  ⚠  ".board-card-header" and ".issues-details-filters (app/assets/stylesheets/pages/boards.scss -55:3)" have the same            stylelint-gitlab/duplicate-selectors
           properties.
 394:3  ⚠  "img" and "svg (app/assets/stylesheets/pages/boards.scss -336:3)" have the same properties.                                  stylelint-gitlab/duplicate-selectors
 464:3  ⚠  ".issuable-sidebar-header" and ".board-header (app/assets/stylesheets/pages/boards.scss -195:1)" have the same properties.   stylelint-gitlab/duplicate-selectors
 476:5  ⚠  Expected ".page-with-contextual-sidebar.layout-page .issue-boards-sidebar .gutter-toggle svg" to have no more than 3 compound selectors                                                       selector-max-compound-selectors
 483:7  ⚠  Expected ".page-with-contextual-sidebar.layout-page .issue-boards-sidebar .gutter-toggle:hover path" to have no more than 3 compound selectors                                                selector-max-compound-selectors
 493:5  ⚠  Expected ".page-with-contextual-sidebar.layout-page .issue-boards-sidebar .issuable-header-text > strong" to have no more than 3 compound selectors                                           selector-max-compound-selectors
 571:1  ⚠  ".add-issues-search" and ".board-card-header (app/assets/stylesheets/pages/boards.scss -363:1)" have the same properties.    stylelint-gitlab/duplicate-selectors
 579:1  ⚠  ".add-issues-list-column" and ".filter-form (app/assets/stylesheets/pages/boards.scss -59:3)" have the same properties.      stylelint-gitlab/duplicate-selectors
 645:1  ⚠  ".modal-filters" and ".add-issues-search (app/assets/stylesheets/pages/boards.scss -571:1)" have the same properties.        stylelint-gitlab/duplicate-selectors

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/boards.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