Allow to filter by IID in Issue Boards

@gimp3695 pointed out in https://gitlab.com/gitlab-org/gitlab-ce/issues/64213#note_195601880 that he'd love to be able to filter issue boards by IID in order to easily drill down to an issue.

This currently does not work because Boards/IssuesController uses IssuesFinder directly, whereas issues/mrs/epics lists go through the IssuableCollections module, which has special code to match IID references and turn them into something IssuesFinder can understand.

In order to support this we might want to consider moving the logic to match an IID search into IssuableFinder