Add non-catch-all version of dashboard route

[ukutaht]

Currently dashboard links are generated with trailing slash across the application. This PR adds a route without the catch-all part which affects the router helpers so the link is generated without the trailing slash as shown here:

iex(6)> PlausibleWeb.Router.Helpers.stats_path(PlausibleWeb.Endpoint, :stats, "plausible.io", [])
"/plausible.io/"
iex(7)> r PlausibleWeb.Router
{:reloaded, [PlausibleWeb.Router, PlausibleWeb.Router.Helpers]}
iex(8)> PlausibleWeb.Router.Helpers.stats_path(PlausibleWeb.Endpoint, :stats, "plausible.io", [])
"/plausible.io"

The trailing slash appears in live demo stats, this is an attempt to suppress it.