gerrit: use !important in commit message font size

Related to bug 44895, 07e32eedf implemented a CSS rule that ended up
being overridden.  Mark it as '!important' to make sure it is taken in
account.

Change-Id: Ifacc7307dbf25ccc4211d9b13943ea949e6230f5
Merge "gerrit: use !important in commit message font size" into production