ref-field should be editable or remove it add-field dropdowns

[harunurhan]

Currently when ref-field which is { "$ref": "link/to/another.json"} is not present, it appears in add-field-dropdown therefore it can be added. However this breaks stuff becauseref-field UI doesn't support editing at this moment.

So we should either allow ref-fields to be added manually or make them editable.

If we decide make it them editable, we should keep in mind that UI should support displaying clickable link or template that is rendered from data fetched from $ref link, in addition to being editable. So maybe having 2 modes view/edit (such as latex) fields could be necessary.

#93 could actually solve this problem also, but requires more effort.