Filter merge requests that are marked WIP

Allow users to filter merge requests by WIP status using the search bar on merge requests list.

Design should be:

The filter should work for group and project merge requests lists and NOT be available for issues list filter.

It is important to remember that merge requests can be marked as WIP using wip: and [wip] prefixes.

Original Description

Would be nice to have a filter on merge requests to show/hide merge requests that are marked as WIP.