Me: Button stacking on narrow screens needs improvement

[v18]

On very narrow screens (< about 384 px, e.g. iPhones in portrait) buttons on /me/security (password) and on /me/security/two-step (enter phone number step) stack poorly:

The layout of the buttons looks correct in an iPhone 6+ in portrait mode, but in smaller phones like the iPhone 5S they aren't adaptive to the available width.

[v18]

Fix #232 when fixing this.