Add db index for post.url column (#2929)

* Add db index for post.url column

As mentioned in https://github.com/LemmyNet/lemmy/issues/2877#issuecomment-1576439829

Not sure if its preferable to do this, or make a combined index
which includes post.nsfw, post.removed, post.deleted

* remove unique

migrations/2023-06-06-104440_index_post_url/down.sql

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+drop index idx_post_url;

migrations/2023-06-06-104440_index_post_url/up.sql

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+create index idx_post_url on post(url);