Logging in returns to the page that was asked for (closes #384)

RequireAuth now sends a logged-out GET to /pages/login with its path
and query in a `next` parameter. The login form carries it as a hidden
field, and a successful login redirects there when it is a path on
this site; anything else, plain or percent-encoded, goes to `/`, which
leads to the webhook list. A browser already logged in that opens the
login page goes to the same place. The navigation bar on the login
page no longer links to the login page.

Model: opus-5-5

internal/handlers/handlers.go

@@ -36,6 +36,9 @@ const (
 	tmplKeyError = "Error"
 	// tmplKeyWebhook is the template data key for a webhook.
 	tmplKeyWebhook = "Webhook"
+	// tmplKeyNext is the template data key for the page to return
+	// to after login.
+	tmplKeyNext = "Next"
 )
 
 // errInvalidPassword is returned when a password does not match.