When editing a snippet: introduce markdown toolbar for markdown blobs
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=247072) </details> <!--IssueSummary end--> When editing markdown snippets at the moment we do not provide the markdown toolbar as it is everywhere else in the product when working with Markdown information (blobs, comments, descriptions). Now, with Editor Lite for snippets, we have proper events listeners that allow us to know whether we are dealing with a Markdown blob or not. | Blob with the toolbar | Snippet | | ------ | ------ | | ![Screenshot_2020-09-09_at_10.56.46](/uploads/1045609f68c68b8df2c05a70e5867fe6/Screenshot_2020-09-09_at_10.56.46.png) | ![Screenshot_2020-09-09_at_10.56.54](/uploads/42418d6b37f03d6f81e5b291f1c8280b/Screenshot_2020-09-09_at_10.56.54.png) | Since Editor Lite is the part of Vueified editing form for snippets, supposedly we could use the `app/assets/javascripts/vue_shared/components/markdown/header.vue` component within `snippet_blob_edit` and toggle its visibility with some flag.
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