Split RSS module into controller + other

[ErisDS]

refs #5091, refs #9192

• This is similar to Split renderChannel into controller + renderer #9218, in that I'm revealing bits of code that are "controllers" in our codebase. As opposed to routes, services, renderers etc.
• This also reveals some code which is identical to the channels controller
• There is more to do here, but for now I've got the module split up, and the tests split and improved.
• Next I'll split RSS into controller + service, DRY up the controller code, etc