fix: Add 10 MB max length checks to form field editor

[frankchn]

Problem

Closes #1814

Solution

As discussed, adds relevant checks for field size on the form field editor itself.

[mantariksh]

frontend code tested and lgtm, but I'm rethinking our decision to remove the backend check. while the check doesn't run on every single update to the form (due to the nuances of findByIdAndUpdate as mentioned in the issue), I think it's still better to have it than to omit it, so it at least runs on the form updates which don't use findByIdAndUpdate. wdyt?

[frankchn]

@mantariksh yeah I think that is fair, undoing the removal.