Get prokeys to work on slack

[GaurangTandon]

Expected compatibilty

• Mathomania
• Auto-inserts
• Multiline snippets with placeholders
• correct caret positioning
• rich text snippets converting to their markdown analogs

List of valid HTML elements inside a Slack p element:

• Emoji (img)
• @mentions (span#URANDOM_ID)

[GaurangTandon]

Current situation:

1. Mathomania works, but when starting off an empty message, every auto-insert has a weird selection: ['[]'] where content in apostrophe is selected.
2. Auto-inserts work, but same problem as above.
3. Nope, they collapse into a single line, and no placeholders work. This is because slack expects all its content to be in a series of p elements.
4. Caret positioning is unavailable as of now :/
5. Nope, does not happen.

[GaurangTandon]

Basically need #259

[GaurangTandon]

We could possibly have a solution just by inserting several paragraph elements and using nextSibling to navigate them.