Evaluate mapping the markdown keystrokes from comment/description fields to Source Editor

At the moment we have several places where Markdown is edited, however there's no parity in how the editing experience works for those in source mode:

• Single-file editor, comments and description fields have the toolbar to add markdown markup
• Comments and description fields also support the keystrokes to wrap the selected text with the matching pair of symbols. However, this is not the case for Source Editor
• Neither WebIDE, nor snippets have the toolbar. Since there's also no keystrokes support, these two categories do not provide any helpers when working with markdown source at all.

One step to fix this is to create the toolbar for Source Editor and add all the buttons available in comments/description/sfe at the moment to that toolbar as part of the existing Source Editor Markdown extension.

Another step (this Issue) is to provide the shared functionality by providing the same set of keystrokes to inject markdown like _, *, etc.

This is not necessarily straightforward task, but in general, I would see it as fitting the following plan:

• Map all keystrokes from /javascripts/lib/utils/text_markdown.js into the Source Editor Markdown extension
• Most probably copy over or replicate (I have a feeling it might be easier with Monaco API than doing raw text update) all the supporting functionality for the keystrokes to work (wrapping the selected text with the matching pair of symbols)

This path even though consisting of only two steps might have some uncertainties