RoutePrefix Handling for LandingPage Form

[bastischubert]

With #283 introduced RoutePrefix the LandingPage form in snmp_exporter has wrong action <form action="//snmp"> resulting in a deadlink

1st reported in prometheus/snmp_exporter#1366

@SuperQ

[SuperQ]

CC @kwilt

[bastischubert]

i guess cutting a new release that includes the already merged changes should solve the issue 🤔