[Annotations] Make 'persistent tags' feature for annotation more explicit

[pafonta]

Persistent tags feature for annotations:

clicking while holding SHIFT will make [a tag listed in the panel 'Annotation tags'] persistent for this paper (i.e., they will be entered by default on every new annotation for this paper). If there are already available annotations for this paper, the user will be asked if the persisted tag should be added to these annotations. This preference is saved locally (i.e., it is not saved with the annotation files and it will not persist between different installation of NeuroCurator). Persistent tags are identified by a light red colouring. Clicking on a persistent tag while holding SHIFT will remove its persistence.

(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.