Add ability to manually discard post drafts

[Oaphi]

closes #1459

Ability to discard a post's draft manually is needed in contexts where the "cancel" button is not available (f.e., when editing user profiles).

Note: as per discussion, the implementation includes:

• a new "cancel" button on the user profile tab that, if clicked, redirects the user back to their profile;
• a markdown tools button for discarding the draft and staying on the page;