Fix UX debt from adapt MR merge widget to new framework

We identified a few inconsistencies and gaps in what was implemented vs the specs:

1. Actions and title should have larger spacing between them (current vs specs).
2. Actions should be ordered from most to least important, both visually and in the markup (image).
   1. Visually, on 768px + breakpoints, actions are right-aligned, with the most important on the right. On <768px breakpoints, the most important action is at the top.
3. The title line-height should be smaller (current vs specs).
4. Details section has different text styles:
   1. The Merge details heading is only shown in the merge blocked states (current vs specs).
   2. The details text should use a smaller font-size (current vs specs).
5. Regardless of breakpoint, when there isn't enough space for the title and buttons, the buttons should go below the title and have some space between them (image).