Improve usability of DarkConsole settings

Summary:
Fixes T3632. Cleans up a bunch of DarkConsole stuff:

• The config setting had out-of-date instructions. Modernize the instructions.
• The setting was sort of hidden under "Display Preferences". Move it to a new "Developer Preferences".
• The setting magically appeared if DarkConsole was enabled on the install. Instead, always show it but explain why it isn't availalbe.
• When the user enables the console, also force it to actually be shown.
• Call out instructions about use of the "`" key more clearly.

Test Plan: Viewed config setting. Viewed settings panel. Changed setting. Enabling the setting showed DarkConsole.

Reviewers: garoevans, chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T3632

Differential Revision: https://secure.phabricator.com/D6594