Add filter to issue boards switcher dropdown

Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/issues/48847 and https://gitlab.com/gitlab-org/gitlab-ce/issues/40219. Originally proposed in https://gitlab.com/gitlab-org/gitlab-ce/issues/40298#note_48542666

One of the biggest challenges currently in issue boards is navigating between multiple boards. We can make this a bit easier by adding a filter input to the boards switcher dropdown:

This is a small effort for a big gain in UX. It does not solve the root problem of duplicated and unused boards, but it definitely alleviates that issue.