refs https://github.com/TryGhost/Team/issues/1174
This paves the way for Ghost to be able to redirect to the referrer
page when dealign with signup magic links. We pass the referrer for
all types of magic links however, to allow extension of this
functionality in the future.
We've also removed the concept of `requestSrc` which has been unused
for a while now.
refs: 23b383bedf
- @tryghost/error constructors take an object, not a string - the expectation is that message, context & help should all be set
- This does the bare minimum and just ensures message is set correctly
