Adds a tiny object for computing the maximum allowable slider value during file upload.  Broke it into its own object to make testing easier, but this may be a target for future refactors.
Image sliders now attempt to access the :slider_max session var to determine what values they should allow.  In the case where no such var exists an error is put at the top of the page.  Moved the actual text of the error into the yaml file so people could fiddle with it w.out messing with the templates.
Merge branch 'feature/context_sensitive_upload_partials' of https://github.com/nu-lts/drs into feature/context_sensitive_upload_partials