Introduce generic type values.NullableValue and reimplement OptionalBool

[lunny]

Purpose

Since all Go basic types are not nullable, and nil has no type for default, but sometimes we need a nullable type. This PR introduces a new package modules/values and a generic type NullableValue which can resolve the defect of Golang types.

What's changed

And this PR also reimplemented util.OptionalBool with the introduced modules/values, the new implementation will only affect the util.go file and less places. Most places reference util.OptionalBool can be kept and works as before.
P.S. A potential bug was found because at the previous implementation util.OptionalBoolNone == 0, so you can always use 0 as that value. But in the new implementation, they are different types.

Next step

If this PR merged, many other places which needs a nullable type could reuse this infrastructure.

[KN4CK3R]

We should decide for one: optional.Option or NullableValue.
Some discussion on the naming of the Option type: #28733 (comment)

[wxiaoguang]

I do not think it needs to introduce a new type, IMO the current optional just works.

[lunny]

OK. I just confused by the function name Some. I will close this one and we can have more discussion about that function name. https://github.com/go-gitea/gitea/pull/29329/files#r1500305151