[Annotations] Make 'persistent suggested tags' feature more explicit

[pafonta]

Persistent suggested tags feature:

clicking on [a tag] in [the panel 'Suggested tags'], while holding SHIFT, [...] will [make this tag] become persistent (indicated by a pale red colouring). That means that they will always figure in the "Suggested tags" list (unless they are already in the "Annotation tags" list). The selection of persistent suggested tags is global (i.e., independent of the paper) and is saved locally only.

(https://github.com/BlueBrain/neurocurator/wiki/Using-the-Neurocurator:-Overview)

A button / check box should be used in NeuroCurator to not rely on the implicit 'clicking while holding SHIFT' behaviour.

Similar to #81.