Search issuables by iids

What does this MR do?

Since !9222 (merged) the IssuableFinder does not look anymore for iids in the search string, but it expects you set the iids through a dedicated parameter.

With this MR I am adding to the issuable controller the ability to recognize an iids in the search string, and replace the search parameter with the iids parameter.

I set the search parameter to nil because they are two filters in cascade: if iids is 3 and search is #3, then it will match only an issue with iid 3 and title containing #3, that of course is not what we want. TBH I think the IssuableFinder should ignore the search if iids is set, but I haven't changed the behavior 'cause I don't want to break the APIs.

Fixes #30974 (closed) and #39908 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers