Hide buttons when logged out

[notama-nr2527]

When loggd out: Hide "Add Data Class" and "Add Rule" & "Export" buttons;
hide "Add Terms" & "Add Relationship Type" buttons, as well as the icons
for rule editing.
The buttons already do nothing since the object they changed is not
accesssible when logged out. Thus, we prevent a spurious error message
to be shown.

Apply Welch's suggestions: add canEdit boolean and fill it apropriately.
Before, we were blindly following the loggedIn status; now we also follow
backend's hints/permissions of whether we should show the button, in
addition to checking if the user is logged in.

[CLAassistant]

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