Ability for groupby-field-based batch actions (rename, delete)

[droumis]

This is one aspect of a larger task: #99

Add the ability in the App GUI to apply an action to an entire category (groupby field) of annotations:

• rename: For instance, if a user has a groupby field called 'category', and a set of annotations with a category of 'A', the action would be to batch rename 'A' to 'B'.
• delete: For instance, if a user has a groupby field called 'category', and a set of annotations with a category of 'A', the action would be to delete all of these 'A' category annotations.