The login page had a broken look on mobile due to a missing selector,

causing some of our style resets for labels to fail to apply. This adds

the missing selector, fixing up the field layout.
The recent changes to button layout also impacted this page, and others

in the UI. The usage of display: inline-flex caused the text to be

left-aligned. This impacted all buttons in the UI with a non-default

width. We now center-justify the content of buttons to restore the

original centering.

Tested the login page on desktop and mobile.
Checked other buttons throughout the UI for regressions.