Fix EZP-23632: Exception when running eZ Publish behind built in reverse proxy

> https://jira.ez.no/browse/EZP-23632

Fixes regression introduced by 9c31418fc70cfb898c2ff3eabd55cd90e14c7b6f (from #1067).

## Explanation
User hash is triggered by an event subscriber which occurs on `kernel.request` with priority `7`, so **after** a few other subscribers/listeners in eZ kernel (e.g. IO listener checking if current URI can be considered as a valid binary file).
Problem was that dependencies of those subscribers/listeners use dynamic settings that weren't updated.

0. User hash request now has a SiteAccess match thanks to the original request (HttpCache)
0. Ensures that user hash request coming from external reverse proxy (e.g. Varnish) also has a SiteAccess match with original request
0. Ensures `/fos_user_context_hash` URI has a route, in order not to trigger `UrlAliasRouter` or `FallbackRouter`
0. Ensures dynamic settings are reset after SiteAccess change
0. Ensures `ezpublish.core.io.image_fieldtype.legacy_url_decorator` uses setter for dynamic setting injection