Remove all IdentityManager references from plugins

Everything now references an IdentityController or IdentityFactory,
and obtains them by injection (or local plugin state) rather than
using a singleton method.

Change-Id: I641e1e0a84f96d928affa310a4e43a42f93d13d4
Reviewed-on: http://gerrit.dmdirc.com/2751
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager