Base path breaks facet autocomplete

Problem/Motivation

We have a site with a non-empty base path, and facet autocomplete works when the base path is empty but not otherwise

Steps to reproduce

Configure a facet on a search API view for select2 with autocomplete. Set up the site so it is served on a base path, eg. https://example.com/mysite. Observe that autocomplete fails and there is an associated error reporting the absence of a matching route.

Proposed resolution

The base path isn't taken into account when processing the URL of the invoking page, and it needs to be. Proposed patch to follow,