require tag to exist on user link creation

lib/philomena/user_links/user_link.ex

@@ -52,6 +52,7 @@ defmodule Philomena.UserLinks.UserLink do
     |> put_assoc(:tag, tag)
     |> put_assoc(:user, user)
     |> validate_required([:user, :uri, :public])
+    |> validate_required([:tag], message: "must exist")
     |> validate_format(:uri, ~r|\Ahttps?://|)
     |> parse_uri()
     |> put_verification_code()

lib/philomena_web/templates/profile/user_link/_form.html.slime

@@ -14,8 +14,8 @@
             | artist:
             em artist name here
           ' or a series name
-      p Should be blank only if your content isn't on the site, generally
       = text_input f, :tag_name, value: assigns[:tag_name], class: "input", autocomplete: "off", placeholder: "artist:name", data: [ac: "true", ac_min_length: "3", ac_source: "/tags/autocomplete?term="]
+      = error_tag f, :tag
 
   .field
     label for="uri"