Allow for User associated tags for studies

[jdkent]

Users may want to organize studies into categories that are conceptually under the same project (set of meta-analyses), but are different in the inclusion/exclusion criteria/how they are annotated/extracted from in the extraction phase. This would allow users to organize their studies into meaningful bins (for them) and label/extract studies in a systematic way where they can apply the same thought process to a group of studies.