Migrate actions from Markdown toolbar to Source Editor

Problem to solve

The markdown editor implemented in issues, epics, and comments as a textarea has functionality built in to help users write GLFM. Some of the most useful tools are the shortcuts available in the toolbar and associated keyboard shortcuts.

In order to even consider the Source Editor as a suitable replacement for the textarea, we need to add the same toolbar actions and related keyboard shortcuts to the Source Editor.

Proposal

Create an extension for Source Editor that can be loaded in the context of editing GLFM files and add formatting shortcuts to the toolbar and keyboard shortcuts for editing Markdown content.

Availability & Testing

Make sure package-and-qa passes. Keep an eye on qa/specs/features/browser_ui/3_create/source_editor/source_editor_toolbar_spec.rb - and if there are any updates required for it.