Don't allow modifying fields that will get overwritten by the next sync

Right now when an extension has been imported from a Maven repository there's a sync button and in the future it'll get synced automatically with a scheduler job. The sync operation overwrites some fields such as: the display name, the summary.

Thus we should make those fields not editable in the UI when editing those extensions so that users can't change them and have their changes overridden.

We could also display a nice message explaining why they are not editable.