Refactor 'copy-to-clipboard.js'

Mike Greilingrequested to merge
es-module-copy-to-clipboard into master
  • replace vendored clipboard.js library with one managed by yarn
  • move copy_to_clipboard.js to behaviors (since it is needed on all pages)
  • fix eslint rule violations
  • convert into pure-module with ES6 export

Related: gitlab-org/gitlab-ce#38869, gitlab-org/gitlab-ce#39072

Edited by Mike Greiling

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