Improve web redirection and 404 behavior.
Improve web redirection and 404 behavior.

Previously we redirected any non-existent path to the root (or path
prefix).

The new behavior:

With no path prefix:

- "" -> "/"
- "/biz" -> 404

With path prefix of "/foo/bar":

- "" -> "/foo/bar/"
- "/" -> "/foo/bar/"
- "/foo/bar" -> "/foo/bar/"
- "/biz" -> /foo/bar/biz"
  (anything not starting with the path prefix gets the prefix prepended)
- "/foo/bar/biz" -> 404

This fixes https://github.com/prometheus/alertmanager/issues/66

jbarratt/alertmanager,abhishekamralkar/alertmanager,Kellel/alertmanager,jbarratt/alertmanager,t0mk/alertmanager,jmptrader/alertmanager,amorken/alertmanager,brancz/alertmanager,novilabs/alertmanager,jbarratt/alertmanager,mrwacky42/alertmanager,tyrken/alertmanager,abhishekamralkar/alertmanager,abhishekamralkar/alertmanager,sarathsp06/alertmanager,tinytub/alertmanager,prometheus/alertmanager,tbouvet/alertmanager,Aloomaio/alertmanager,GoogleCloudPlatform/alertmanager,tyrken/alertmanager,sarathsp06/alertmanager,wrouesnel/alertmanager,MonsantoCo/alertmanager,jbarratt/alertmanager,joehandzik/alertmanager,prometheus/alertmanager,jmptrader/alertmanager,tinytub/alertmanager,joehandzik/alertmanager,brancz/alertmanager,GoogleCloudPlatform/alertmanager,dylanmei/alertmanager,Aloomaio/alertmanager,dylanmei/alertmanager,jmptrader/alertmanager,MonsantoCo/alertmanager,abhishekamralkar/alertmanager,novilabs/alertmanager,jmptrader/alertmanager,wrouesnel/alertmanager,joehandzik/alertmanager,Aloomaio/alertmanager,GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,tbouvet/alertmanager,brancz/alertmanager,pdf/alertmanager,Kellel/alertmanager,pdf/alertmanager,amorken/alertmanager,dylanmei/alertmanager,dylanmei/alertmanager,brunoqc/alertmanager,t0mk/alertmanager,brancz/alertmanager,prometheus/alertmanager,novilabs/alertmanager,MonsantoCo/alertmanager,brunoqc/alertmanager,amorken/alertmanager,brunoqc/alertmanager,tyrken/alertmanager,tyrken/alertmanager,ericgerling/alertmanager,novilabs/alertmanager,wrouesnel/alertmanager,ericgerling/alertmanager,mrwacky42/alertmanager,pdf/alertmanager,wrouesnel/alertmanager,brunoqc/alertmanager,t0mk/alertmanager,Kellel/alertmanager,amorken/alertmanager,pdf/alertmanager,tbouvet/alertmanager,MonsantoCo/alertmanager,tinytub/alertmanager,mrwacky42/alertmanager,mrwacky42/alertmanager,sarathsp06/alertmanager,ericgerling/alertmanager,t0mk/alertmanager,prometheus/alertmanager