Reordering Transcription/Metadata fields broken after lines are deleted

[benwbrum]

If a project owner has a field-based project (or metadata description project), they are not able to correctly drag fields from one line to another if they have deleted all the fields from a preceding line.

[benwbrum]

To reproduce:
Video

[benwbrum]

The underlying problem seems to be that we store the field position in transcription_field.line_number in absolute form, but the drag-and-drop code calculates the drop target's line number based on its relative position. If all fields from line 1 (position 0) have been deleted, the underlying data for line 2 and 3 remains 2 and 3, but the drop targets for those lines will be position 0 and 1. (Positions are zero-indexed while lines are 1-indexed.)

One approach might be to make the controller re-number the fields in the underlying data after any field deletions (or emptying a line by dragging fields out of it). Another approach might be to make the view aware of the line_number values somehow.