Improve post and comment composition TextField

[buresdv]

Give users the ability to insert Markdown snippets to insert pictures, styling, etc.

For this to work:

• The Textfield needs to keep track of its cursor postition so we can manipulate it when adding the Markdown markup
• The Textfield needs to be able to manipulate the cursor position
• The Textfield needs to have identical styling to the current one
• The Textfield contents need to be bound to the .toolbar above the keyboard, which will include buttons for adding the various Markdown markup