Exclude terms from cell-level characteristics when suggesting terms for completion

[arteymix]

Terms that are loaded in cell type assignlemnt and as cell-level characteristics should not be suggested for factor values.

This is a bit tricky to implement because Hibernate does not surface foreign keys in the Characteristic model. It would have to be done in raw SQL.

A good approach would be to only display terms that are attached to other FactorValue objects. For experiment-level tags, only experiment-level terms should show up.

[arteymix]

This can be done in the next patch release since it can be implemented by only including expeirment and factor value-level characteristics in the combo box.