Replace hardcoded <tt> HTML tags with <kbd>/<samp>/<code> tags
Summary:
<tt> is deprecated in HTML5 and should be avoided: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/tt
Replace the hardcoded <tt> elements with semantically more appropriate alternatives and update our CSS accordingly.
Note: This change does not handle/affect <tt> elements created by Phorge in a Remarkup context (`tt.remarkup-monospaced`).
Test Plan:
* Go to http://phorge.localhost/uiexample/view/PHUITimelineExample/ and look at the top box
* Go to http://phorge.localhost/config/issue/ and open an issue which shows a command in its description (e.g. "Daemons Are Not Running")
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26671