User preferences ported from tools

Summary:

Internal tools, e.g., differential and diffusion have user defined
preferences for monospaced font and the option for showing either the
name of the tool or the glyph of the tool in the title.

These preferences were ported to phabricator. These preferences can be
modified in /preferences/ and they both affect diffusion and differential
at the moment.

Test Plan:

• Created an empty database
• Loaded /preferences/ and modified the monospaced font and clicked save
• Confirmed that the same page was loaded with the message that preferences have been saved and that the example text used the user defined font

• in /preferences/ changed the option to show tool names as plain text and clicked save
• Confirmed that the same page was loaded with '[Preferences]' in the title instead of a glyph

• These same tests were also executed for differential and diffusion

Reviewers: epriestley
CC: jungejason

Differential Revision: 91