Add a way of seeing the referrer for each 404

Thanks @DamienMcKenna I totally agree.

Surely this would have to be tracked in another table and as additional information, but knowing the referrer is extremely helpful to find out and fix the 404's. That way we can wee, where the wrong link is coming from.

It's a 1:n relation, so it definitely needs another table. I guess the current 404's list can be kept and a link to details should be added, when clicking an entry. That one should then list the referrers.

Clearing the 404's should also clear that additional table.

Might be a nice feature to implement for Novice's?

After applying the MR we need to use the drush updb command once.
The referrers will be stored in the 404_referrers table.
Too see the referrers from UI itself
1. Visit /admin/structure/views/add
2. Under View Settings select 404 Referrers
3. Add necessary fields to view the referrer