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;
• an indicator (analogous to the "draft saved" one) for when a draft is successfully discarded;

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.25%. Comparing base (a9400ee) to head (79ee877).
⚠️ Report is 22 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	70.87% <ø> (ø)
helpers	81.96% <ø> (ø)
jobs	60.00% <ø> (ø)
models	88.36% <ø> (ø)
tasks	61.11% <ø> (ø)

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[cellio]

LGTM with one small tweak to a tooltip.