Add ability to search all MRs inside a project

In version 1.0, we currently fetch the most recent 100 MRs and allow users to filter through those. Because of Trello API limitation and iteration speed, we chose not to allow users to search the gitlab API for any MRs within their selected project.

We should determine when Trello plans on updating their API to support this endeavor and also determine whether it makes sense to roll our own view to enable such a feature

/cc @filipa

Trello team says they are actively working on adding debounce support

Debounce just shipped: https://developers.trello.com/v1.0/reference#section-popup-search