base: Easier debugging at page load with sessionStorage

This is more straightforward than screwing around with cookies.
Updated instructions too.

Closes #1611
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
ws: Forward "close" control messages properly to the bridge

These should be forwarded as the last message of that channel. Do
the session lookup before removing the channel from the table.

Closes #1610
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
common: Add cockpit_json_get_array.
bridge: "metrics1" channel.

Initial work by Stef Walter <stefw@redhat.com>.
shell: Use metrics channel for dashboard plots.
FIXUP - install libpcp in travis.
FIXUP - -Werror
WIP - test skeleton
WIP - mock pmda