Settings - upgrade monospace font regexp to support '.'

Summary: this data is a little weird since its user-entered and we need to put it in a web page un-escaped for the font to load correctly. Ergo, we use a regex to make the input safe / sane, and said regex needs to support a '.'. Fixes T5810.

Test Plan: added Fixedsys Excelsior 3.01 to my system and was able to set my preference and get the new font

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: dereckson, epriestley, Korvin

Maniphest Tasks: T5810

Differential Revision: https://secure.phabricator.com/D10163