Bug 22053: ability to enable/disable some plugins

Bug 22053: ability to enable/disable some plugins

Test plan:

  - apply this patch(es),
  - launch an updatedabase,
  - go to plugins/plugins-home.pl
    and deal with enable/disable method
  - install a plugin like KitchenSink
    https://github.com/bywatersolutions/koha-plugin-kitchen-sink
  - once installed, the plugin change the background color
    of the staff client to orange.
  - disable the plugin,
  - background color should be set back to the original one

Rebased-on: 2019-03-25 Alex Arnaud <alex.arnaud@biblibre.com>

Signed-off-by: Claire Gravely <claire.gravely@bsz-bw.de>
Signed-off-by: Agustin Moyano <agustinmoyano@theke.io>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>