Allow filtering issues by any assignee

[asvanberg]

This is the opposite of the "No assignee" filter, it will match all issues that have at least one assignee.

Before

After

[wxiaoguang]

If I understand correctly, this PR only changes the db search logic.

But there are other search engines, would they go wrong? Correct me if I was wrong.

[asvanberg]

If I understand correctly, this PR only changes the db search logic.

But there are other search engines, would they go wrong? Correct me if I was wrong.

I searched the code base to find where there was special logic for the db.NoConditionID relating to issue assignees and only found that place and changing it appeared to have solved it.

Looking at modules/indexer/issues/dboptions.go I can see that it converts the -1 (db.NoConditionID) to 0 before passing the search options to the indexers. I could attempt to propagate a magic value down to the indexers search options and handle it in bleve (and the other indexers too I assume) but if you don't want to keep using magic numbers that seems like it would just make a future migration to another datatype require more work?

[wxiaoguang]

Without a complete refactoring, I could accept a non-conflicting magic number other than -2 at the moment (see the NonExistingID below), and I think it needs some tests to cover the behavior.