Support protected branches when migrating project

[lorenzo-cavazzi]

Motivation

We don't fully support protected branches everywhere in the UI, but we should handle better a few cases since renku-core actually supports that case.
Currently, updating a project with protected branches using the "Update" button from the settings page seems to work but actually leaves the user with the same update button.

Proposal

When migrating a project with a protected branch, we should show a warning and a link to a merge request on GitLab.
This should be easy since renku-core return an ad-hoc response in that case containing the field remote_branch.

{
   "result":{
      "docker_migrated":true,
      "errors":[
         
      ],
      "messages":[
         "Updating dockerfile...",
         "Updated dockerfile.",
         "Applying migration m_0009__new_metadata_storage...",
         "Applying migration m_0010__metadata_fixes...",
         "Migrating old namespaces",
         "Fixing activity ids",
         "Rebuilding workflow metadata",
         "Successfully applied 2 migrations."
      ],
      "remote_branch":"renku/autobranch/master/d1fadcfa",
      "template_migrated":false,
      "warnings":[
         
      ],
      "was_migrated":true
   }
}

[lorenzo-cavazzi]

Not relevant anymore for Renku 2.0.