Sometimes we define URLs for POST requests which are not defined for GET
requests, such as "/residence", so redirecting to it after signing out
results in a routing error.
So instead of using the request referer, we're using the stored location
devise uses, and we're not storing locations in POST requests.
For reasons I'm not sure about, the homepage (and the welcome pages)
were an exception in our "redirect users to the same page they were"
policy.
I'm not sure about the welcome pages (no test was present indicating
they should behave in a special way), but in the case of the home page,
it was a bit annoying to be redirected to a different place after
signing in.
When signing in from a page containing GET params, like
`/budgets/1/investments?heading_id=4`, we were redirected to a URL
without those GET params; in this case, `/budgets/1/investments`.
Using the request fullpath, as recommended in the devise documentation,
keeps these parameters when redirecting.
