Re-invoke the config.show API when changing reference on the Start session page

[lorenzo-cavazzi]

Motivation

When renku-core supports passing references to the GET /config.show endpoint (see SwissDataScienceCenter/renku-python#3649), we should update the code to start sessions so that selecting a new branch/commit also triggers a new query to update the config for the project.

Proposal

It should be easy once we can pass the references to the endpoint as a query parameter branch=<commit-sha>.
Remember to remove the logic to query the specific file from GitLab.