report: refactor UI string logic

[paulirish]

cf. #5719

• all uses of Util.UIString in the renderer pass through Util.str()
• this fn will use the supplied localized string, if that was provided. otherwise it'll fall back to the original string in the util.js source file. (which is en-us)
• no more mutate/restore dance on this data

[patrickhulce]

works for me!

anything spark this? not sure I see it being fundamentally different than the mutation 💃

[patrickhulce]

does it have to be str? :)

I get that we don't want it to be too similar to i18n ones since it's quite different, but maybe a bit more descriptive since we're already forced to wrap :D

[brendankenny]

at some point we'll want to refactor our global stuff, but not today :)