Mirror comment field in ADD EXTERNAL modal

Add a new free text comment field in the ADD EXTERNAL DB modal that mirrors the main comment field under EXTERNAL DB on the classification page. I.e., text edits in the new modal field should be shown in the EXTERNAL DB main comment, and vice versa.

This is both to avoid misuse of the free text field connected to "Other" in the modal (currently used by many for comments) and to make commenting more convenient,

NB: This solution is deprecated if #47 (moved) is implemented, so should only be considered a temporary fix.

Original user request: MEET-139