Replace :-moz-placeholder CSS pseudo-class with ::-moz-placeholder pseudo-element
Summary:
Per https://caniuse.com/css-placeholder, `:-moz-placeholder` was supported up to Firefox 18 released 11/2012.
Firefox 18 to Firefox 50 (released 11/2016) support `::-moz-placeholder` instead.
We currently already use the latter in `phui-form-view.css` for `.aphront-form-input`.
This is a followup to rP8353146b; we missed this corner-case at that time.
Test Plan: None. Probably installing old browser versions if you don't trust documentation.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26339