Prevent undepositable subjects beting unset when eprint is edited

[drn05r]

When a subjects type field is being displayed as a Field::Subject component then subjects that have been made undepositable still show as being set but if a standard component type is used then undepositable subjects are not display and if the user clicks through to the next stage these subjects will be unset without the user realising, which is undesirable.

Behaviour should be changed so the standard component type behaves the same as the Field::Subject component type and still displays undepositable subjects (at the top of the field) if they are currently selected for that item.

[drn05r]

This has a number of issues:

1. It does not handle subject fields that are not multiple.
2. It does not check whether the values already selected are depositable or not so select values will be displayed twice.

[drn05r]

Latest commit should fix the two bugs described.