URL customisers applied for all platforms

Overview:

URL customisers are being retrieved and applied for all platforms, and not just the ones where they have the platform as the context

Steps to Reproduce:

1. Find a platform and add a URL Customiser
2. Retrieve all customisers for a different platform using /erm/sts/template/<platform UUID>

Expected Results:

Only customisers for which the platform is a context is retrieved

Actual Results:

All customisers retrieved

Additional Information:
This issue seems to extend not just to retrieval, but then the application of the customiser- all the resources receive the customiser even though it is meant to apply only to a single platform (unlike a proxy which would apply to all PTIs except those on the platforms in the contexts)