Require the "administrate server" core capability

Capability scope was introduced to differentiate between plugin-owned
capabilities and core capabilities. Per default the scope of the
@RequiresCapability annotation is CapabilityScope.CONTEXT, which means
the scope is assumed to be that of the plugin.

Update the annotations to use CapabilityScope.CORE, to make sure the
proper "administrate server" capability is used.

Change-Id: I02d3ed77f6e7dee746686b5ec8711aa38dddd9c3