Add back and redirect helpers.

[CrowdHailer]

The function Raxx.redirect and Raxx.back should exist.

Can be pulled from Tokumei implementation https://github.com/CrowdHailer/Tokumei/blob/6d4d88c0c5e485d79c0fb357f6730f2cf799420b/app/lib/tokumei/helpers.ex

Or sugar https://github.com/sugar-framework/sugar/blob/405256747ce8c446c504e4dc533b24c76d864a5a/lib/sugar/controller/helpers.ex#L253-L259

QUESTION: Does this belong in a Raxx.Browser utility?

[CrowdHailer]

Fixed in PR #96