Correct a mask config value

Summary:
The correct name of this key is 'github.application-secret', not
'github.secret'. Make DarkConsole check that all the masked keys exist to
prevent this from happening again. This isn't super important since this
is just intended to protected against casual security lapses (taking a
screenshot with DarkCnosole's "Config" tab open, for instance) but it's easy
to check for so it seems worthwhile to get right.

Test Plan:
Loaded page without the actual config file change, got an exception.
Fixed the config, reloaded the page, good news goats (really trying to get this
to catch on since goats are adorable).

Reviewed By: aran
Reviewers: tuomaspelkonen, jungejason, aran
CC: aran
Differential Revision: 189