Rework web interface to be workspace centered

This issue is about changing the structure of the current web interface to be centered around workspaces as described in https://freexian-team.pages.debian.net/debusine/reference/devel-blueprints/monitor-workflows.html#expected-changes

Basically it implies to rework the URL structure around some sort of /<scope>/<workspace>/ prefix for most views.

• current workspace is shown in menu bar (and has a drop-own to switch workspace) (#624 (closed), !1450 (merged))
• list of work-requests is at /<scope>/<workspace>/work-request/ and is restricted to the current workspace (#584 (closed), !1362 (merged))
• inside a workspace, menu bar has a "Workflows" menu (#620, !1428 (merged), !1495)
• inside a workspace, menu bar has a "Collections" menu (!1436 (merged))
• "Workers" and "Task status" should be moved out of the menu in the footer on a "Status:" line (!1416 (merged))
• "Create artifact" and "Create work request" should be workspace specific (#585 (closed), !1362 (merged))
• /<scope>/ should list the workspaces available in that scope that are visible to the current user (!1433 (merged))
• /<scope>/<workspace>/ is extended to show figures about workflows (possibly after !1428 (merged) to reuse display code) (#654)

Note that each of those items has more details in the development blueprint, please refer to the development blueprint and not only to the short description.

Time spent on this issue should be reported in #400 (closed).