Fixed MR widget content wrapping for XS viewports

Centers the "Accept Merge Request" button within the MR widget (and adds a bit of right/left padding) for viewports smaller than 768px.

BEFORE:
![Screen_Shot_2016-12-02_at_8_53_01_AM](/uploads/fcdae5302a1407090483dd3cff868c51/Screen_Shot_2016-12-02_at_8_53_01_AM.png)

AFTER:
![Screen_Shot_2016-12-02_at_9_43_37_AM](/uploads/f7d1dce082b79d263f05cea3c90e870a/Screen_Shot_2016-12-02_at_9_43_37_AM.png)

cc: @markglenfletcher @annabeldunstone 
Closes #25202

See merge request !7900