Fix pressing "Enter" in the "add link" modal submitting the parent form

[Oaphi]

closes #2023

This is a "quick and dirty" version of the fix as I don't have full access to my usual tools at the moment (a proper fix would be moving the modal outside the markdown_tools partial as form tags can't be nested, wrapping the inputs in a <form> tag, and cancelling [preventDefault] any submit event on it - see the image_upload partial for how it should be done). Nevertheless, it should do the trick.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.76%. Comparing base (39d0527) to head (d712429).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	75.87% <ø> (-0.04%)	⬇️
helpers	85.09% <ø> (ø)
jobs	66.88% <ø> (ø)
models	93.01% <ø> (ø)
tasks	61.11% <ø> (ø)

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[cellio]

Works now -- pressing enter in either field (anchor text or link) does nothing, so you have to click the "insert" button to return to the editor. No premature post submissions.