Bug 20415: add Koha.ArePluginsEnabled Template Toolkit method

Bug 20415: add Koha.ArePluginsEnabled Template Toolkit method

In order to remove UseKohaPlugins syspref, adds to Koha Template Toolkit
plugin a method ArePluginsEnabled().

Test plan:
1) Set syspref UseKohaPlugins enabled
2) Set config enable_plugins=1
3) Check you see "Tool plugins" in "Tools" home page and menu
4) Check you see "Manage plugins" in "Koha administration" home page
5) Check you see "Report plugins" in "Reports" home page and menu
6) Set config enable_plugins=0
7) Check you don't see in 3) 4) 5)
8) Run prove t/Koha_Template_Plugin_Koha.t

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>