Normalize remote address before saving to context

One cause for https://todo.sr.ht/~sircmpwn/meta.sr.ht/193 was that code
performing this sanitization was in multiple places, and a fix was not
applied in all places. There is no reasonable expectation for the port
to be present anyways, e.g. if the address is taken from a trusted
reverse proxy's header. Hence, perform the normalization here, so that
the code doing this in applications can be simplified.

Note that this does not yet fix the below ticket, it will just make the
fix easier.

References: https://todo.sr.ht/~sircmpwn/meta.sr.ht/193

server/server.go

@@ -193,8 +193,16 @@ func (server *Server) WithDefaultMiddleware() *Server {
 	server.router.Use(middleware.Timeout(timeout))
 	server.router.Use(func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			var err error
+			addr := r.RemoteAddr
+			if net.ParseIP(addr) == nil {
+				addr, _, err = net.SplitHostPort(addr)
+				if err != nil {
+					panic(fmt.Errorf("Invalid remote address: %s", r.RemoteAddr))
+				}
+			}
 			ctx := context.WithValue(r.Context(), serverCtxKey, server)
-			ctx = context.WithValue(ctx, remoteAddrCtxKey, r.RemoteAddr)
+			ctx = context.WithValue(ctx, remoteAddrCtxKey, addr)
 			r = r.WithContext(ctx)
 			next.ServeHTTP(w, r)
 		})
@@ -203,6 +211,8 @@ func (server *Server) WithDefaultMiddleware() *Server {
 	return server
 }
 
+// RemoteAddr returns the remote address for this context. It is guaranteed to
+// be valid input for `net.ParseIP()`.
 func RemoteAddr(ctx context.Context) string {
 	raw, ok := ctx.Value(remoteAddrCtxKey).(string)
 	if !ok {