fix: prevent '//' in URI paths created with JSONModel
The uri_and_remaining_options_for method now strips excess '/' characters when joining the sub-path to the base path. This prevents Jetty rejecting HTTP requests to the backend due to ambiguous empty path segments.
For example, the frontend repositories_controller is currently creating such a URL containing '//' with this call:
@enum = JSONModel(:enumeration).find("/names/archival_record_level")
This fixes: https://cambridge-collections.atlassian.net/browse/AR-406