After [40059] the CSS class `button-link` uses `text-align: left` by default.
This change now requires to limit as much as possible the use of `button-link`
to controls that should really look like links and to explicitly set
`text-align: center` in a few other cases.
Fixes#39983.
Built from https://develop.svn.wordpress.org/trunk@40358
git-svn-id: http://core.svn.wordpress.org/trunk@40265 1a063a9b-81f0-0310-95a4-ce76da25c4cd
