Fix contributions override from imported bundles on 9.10

Use case:
In Designer, an html bundle file is generated to hold all the UI contributions. When the Studio package is installed, Web UI loads that html bundle file (by convention) and renders the contributions. In this html bundle file, additional html files can be imported to include supplementary contributions.

Problem:
On 9.10, contribution overrides coming from imported bundles are discarded (regardless of priority). If the contribution on the imported bundle file has a priority higher than the contribution in the main bundle (or even a default one), than it should be displayed instead of the others. If the contributions are not overrides, they work normally (as if they were included in the main bundle). This behaviour doesn't seem to be reproducible on 8.10, 10.2 and 10.3, only on 9.10 and 10.1.

Reasoning:
After NXS-4758, Designer users are no longer advised to use the project bundle html file to introduce their custom code. Instead, they are encouraged to add it to a separate bundle file (custom code bundle), that is imported on the main html bundle file (project bundle).

Steps to reproduce:
A jar file has been attached with a basic example:

• Add the jar file to a 9.10 server with nuxeo-web-ui (in nxserver/bundles/)
• Start the server and notice that a new DOCUMENT_ACTIONS action was added with a "cross" icon and a "low priority" label.
• Examine both nuxeo-mnixo-SANDBOX-bundle.html and nuxeo-mnixo-SANDBOX-custom-bundle.html files (in nxserver/nuxeo.war/ui/) and verify that a DOCUMENT_ACTIONS action with a "check" icon and a "high priority" label should be shown instead.