Change XHR request URL to base CP URL

[bencroker]

Front-end action URL requests are treated differently than control panel action URL requests. Snaptcha, for example, blocks all POST requests to the front-end that do not contain a validation field value. CP requests however are not validated because the user is assumed to anyway be authenticated.

Since XHR calls to searchit/element-filters/get are valid for the CP only, this pull request changes the XHR request from the front-end URL to the base CP URL for more consistent behaviour. I hope you'll consider it :)

[john-henry]

+1 for this request. It's really stopping clients using plugin as filters don't appear in most cases