Bug 20383: Hide link to plugin management if plugins are not enabled

Bug 20383: Hide link to plugin management if plugins are not enabled

This patch adds a check in admin-home.pl to see whether plugins are
enabled. If plugins are disabled, the link to plugins management is
hidden even if the user has plugin management permission.

To test, view the administration home page with plugins enabled and
disabled via the enable_plugins flag in the Koha configuration file.
Also test using users with different plugins permissions:

- Plugins enabled
  - CAN_user_plugins = 1
    -> Plugins link appears
  - CAN_user_plugins = 0
    -> Plugins link hidden

- Plugins disabled
  - CAN_user_plugins = 1,
  - CAN_user_plugins = 0
    -> Plugins link hidden

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>