Fixes for featured (pinned) links

[Oaphi]

closes #1726

This PR fixes two tightly-coupled bugs that we have with pinned links:

• it's now possible to create pinned links without post id (as expected);
• creation of pinned links no longer silently fails for invalid links;
• updating pinned links no longer silently fails when trying to save invalid updates;
• we now validate that either a post or a URL is set;

---

This is how attempting to create an inavlid pinned link looks like now:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.66%. Comparing base (8f917b1) to head (e8b5f5f).
⚠️ Report is 10 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	68.95% <100.00%> (+0.24%)	⬆️
helpers	79.01% <ø> (ø)
jobs	48.57% <ø> (ø)
models	85.90% <100.00%> (+0.03%)	⬆️

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