Reworked Required and OmitEmpty #8

The old behaviour is inconsistent because this fails:

type TestForm struct {
	Valid string `binding:"Url"`
	Fails string `binding:"Email"`
}

form := &TestForm{
	Valid: "",
	Fails: "",
}
_ := RawValidate(form)

Here you need OmitEmpty even if the field is not required.

The new logic:

Empty	Required	Valid
Yes	Yes	No
Yes	No	Yes, all other rules are skipped
No	/	Maybe, other rules decide

In short: Rules are skipped if a field is empty and not required. Thats the normal behaviour in other validation libraries like Yup too.

After this change there are some places in Gitea where we can add form validation to optional fields which is not possible at the moment. Possible changes in:
21465a2ce3/modules/structs/admin_user.go (L31)
21465a2ce3/modules/structs/admin_user.go (L35)
21465a2ce3/modules/structs/org_team.go (L38)
21465a2ce3/modules/validation/binding.go (L90)
21465a2ce3/modules/validation/binding.go (L108)

Please resolve the conflicts.

I see no conflicts? There was a conflict which should be resolved with the merge of the master branch some hours ago.

It maybe a bug of Gitea, when update, it displays below error.

Auto-merging validate_test.go
CONFLICT (content): Merge conflict in validate_test.go
Auto-merging binding.go
Automatic merge failed; fix conflicts and then commit the result.

@KN4CK3R can you rebase or update this pull please?

Reworked Required and OmitEmpty #8

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