This adds a new platform admin settings row, leading a page which
shows any existing keys and allows a new one to be registered. Until
the APIs for this are implemented, the user API client just returns
some stubbed data for manual testing.
This also includes a basic JavaScript module to do the main work of
registering a new authenticator, to be implemented in the next commits.
Some more minor notes:
- Setting the headings in the mapping_table is necessary to get the
horizontal rule along the top (to match the design).
- Setting caption to False in the mapping_table is necessary to stop
an extra margin appearing at the top.
