In Iddef0805ab a custom message store was introduced for ULS, but
there were updates in jquery.i18n that allows us to use default
message store again.
(see https://github.com/wikimedia/jquery.i18n/pull/40)
Change-Id: I86cb7a44efa83e5811824cd1104c6be11b1e2925
Instead of using the whole jquery.i18n code, use only required parts
that are not customized in MW ULS. For this define a new RL module
ext.uls.i18n.
Also refactor MW message store for jquery.i18n.
Change-Id: Idda9fe505428ade409d937314d7216ebb4232c36