Update merge block widget to show all reasons why merge request can not be merged

Background

Based on the discussion in issue #379194 (closed), we did a design to see all merge check failures. The design has been tested and discussed; details see the solved discussion in design

Optional(If this is not too much work): remember the user's status of collapse status for block reasons. Ex: if a user collapsed on block reason and open a new tab, we shouldn't expand by default anymore. We should remember the users status