Fix/unify usage of redirects

Baseline: status as of #2670118-24: Make getPostLoginDestination / getPostLogoutDestination configurable with
* no_cache: TRUE added to the routing.yml (for links both before and after the redirect to the IDP);
* a large comment inserted somewhwere.

@seanB: can you try out this patch? You'll probably need to patch the -dev version with this issue, in order to get the redirect to your external site to work. (Because I removed the following from the -dev code: #2670118-9: Make getPostLoginDestination / getPostLogoutDestination configurable. So you are the dedicated guinea pig now.)

Done:

• Do not let the PHP toolkit do redirects; do it ourselves. Then we can have more influence on how to do redirects (even though it might not be necessary). Credit droath.
• Unify code: replace the other RedirectResponse objects by TrustedRedirectResponse too.
• Move all that into a helper function with a boatload of comments. (The amount of comments is influenced by the fact that I still don't get all the internal details, but I've stopped caring.)

Renamed redirectResponseFromUrl() to createRedirectResponse(). ('redirect' is also a verb.)

Interdiff is from 1 to 3.

Thanks all! Finally committed.