browse: Simplify metadata popup and hide toggle button when not needed
Rework the content of the popup than can be shown when clicking on the "Show metadata" button by only displaying raw metadata associated to a swh object.
As only revision objects can have raw metadata associated, hide the toggle button in views dedicated to browse other object types. The button will also be hidden if revision metdata are empty.
Closes #2848 (closed)
Migrated from D4659 (view on Phabricator)