shell: Remove unused code from journal page

The search code is no longer in use and is rather complicated to
maintain going forward. The serivce filtering is not used by the UI
but is a vital part of tests: simplify it.

Add tag based filtering (ie: logger --tag) which is also used
from integration tests, and not exposed in the UI.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
shell: Pass a string or array as command for cockpit.spawn

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
shell: Implement a journal loading based on journalctl

Use journalctl to load and then follow the journal, by having it
output json and send it to us.

Fixes #850
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
daemon: Remove journal Query code from cockpitd

Closes #989
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>