Add SECRET_KEY_PLACEHOLDER as secret key

After much deliberation, I've decided that it would be too annoying
to try to auto-generate the SECRET_KEY for every app every time, and
asking each user to setup their own SECRET_KEY would put too much burden
on the user.

Our static app will never need a SECRET_KEY as long as it remains static.
If a database is ever introduced, hopefully the SECRET_KEY contents
(SECRET_KEY_INSECURE_PLACEHOLDER) will be obvious enough that it
won't remain unchanged for long.