Formatting of system notes in merge request widget

There are a number of styling problems with the paddings and margins of system notes:

Not yet merged and one system note

Padding to the top is 8px, to the bottom it's 16px, which causes the system note to be off center (hovered over the grey bar to make it stand out more).

Not yet merged and multiple system notes

Same problem here, the bottom padding is inconsistent with both the top padding and the padding between system notes.

Merged and one system note

Due to the fact that the system note is not part of .mr-widget-body, there is a big gap between "The source branch has been deleted" and the Jira mention.

Merged and multiple system notes