Redirect from `/tokens` path

[manuelwedler]

When directly accessing explorer.raiden.network/tokens this error is returned: Cannot GET /tokens.
It only works if a token address is specified in the path (/tokens/0x6B175474E89094C44Da98b954EedeAC495271d0F).
We should redirect to the default route, as it is common mistake to open the /tokens route.