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;

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.35%. Comparing base (e5d4774) to head (fe39f71).

Additional details and impacted files

Components	Coverage Δ
controllers	59.24% <ø> (ø)
helpers	61.95% <ø> (ø)
jobs	28.00% <ø> (ø)
models	57.17% <ø> (ø)

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