Fix double validation of allowed URLs

In some situations, like in Data Adapter forms, we check twice if a URL is in the allowed list. This causes a visual jump and also wastes some resources, as you can see here:

> https://user-images.githubusercontent.com/122591/139851969-7af84874-1dd5-4385-ab86-f5820ea90a6c.mp4

> If you can fix this double update when the URL is changed, that would be awesome. But if it's not feasible within this PR, then this all looks good!

> _Originally posted by @kyleknighted in https://github.com/Graylog2/graylog2-server/pull/11407#pullrequestreview-795299257_

We can probably fix the double validations on this particular case with a flag but after fixing a couple other validation issues with URL allowed lists and Lookup Tables, I think we could benefit from taking a step back and see if there is an underlying issue that we can tackle to make the code less error-prone.