Account overview: list & details page

[nhoening]

We have no page to show an account.

Inspiration can be taken from the CLI commands show accounts (list accounts) and show account (show roles, assets and users of one account).

For now, the account page only shows, later we'll make it possible to edit things here, as well.

This new page should be linked from the user list and the current user page.

Also, the main menu entry "accounts" should appear for admins (and link to the account list). The "users" entry should appear for everyone else (as it shows the users in their account).

We'll want to work towards account CRUD (maybe we'll never get to the D in the UI, but still), so I suggest we set up the usual workflow: Begin work on a real API endpoints for accounts (to list them & get one specific), which are then used by the UI.