TVB-2775. Avoid imports of werkzeug in core (FilesHelper)
Created by: popaula937
Moved methods that were used only in the REST layer more closely to their usages. Having a dependency of werkzeug at this level, makes our H5 layer a bit heavy to use in external modules, eg: tvb-multiscale need to install werkzeug library only for this. This PR should keep our H5 layer more lightweight.