List scope instead of workspaces in homepage

Homepage is scope-less, but lists (all) workspaces, which calls reverse('workspace:detail') without scope, linking workspaces from non-default scopes to the default scope.

Proposal: split the current homepage into:

• a list of scopes that goes in /
• a list of workspaces that goes in /scope/ (cf. !1187 (merged)).

Also filters the scopes that the current user has access to (depends on !1187 (merged)/!1207 (merged)/!1222 (merged)).

(report time in #395 (closed))