Add icontains support to IntegerField

[crackeroil]

No description provided.

[stefanmajoor]

On a table with 67 million entries, this is very slow. And since you expose it to the end user, it creates an endpoint to potentially bring down the server.

My proposal would be to add something that we can set extra filters on a per field basis. Then you can add it to fields for which it is safe (or which have a proper index).

[knokko]

Closing in favor of #281

I am going to ignore Stefans performance concern because this PR is still useful for smaller tables.

Furthermore, even without this PR, there are plenty of ways for users to create slooooow queries (e.g. querying an api of a very large table without any filters at all). I think restricting access to such large tables is the only way to resolve such performance concerns (or by not creating any view at all for such large tables).