minor bugfixes to 2.20 release

• pull in alert variables from env
• show correct message on front page

edit:

Clarifying the changes in this patch:

• dispatching email and phone alerts is controlled by two variables: SEND_PHONE_ALERTS, SEND_EMAIL_ALERTS that were hard-coded into each tier's (production, playground, test, dev) individual settings.
• these variables were supposed to control the informational "enabled"/"disabled" message on the front page of gracedb but.... it didn't ( "broken").

This patch, in conjunction with a tweak in the deployment stacks moves these variables to the deployment as environment variables (so alerts can be switched on/off without updating the server code), but it also fixes the messages on the front page. I tested this on gracedb-dev earlier today to confirm that it works.

Given that: * the core functionality is unaffected, and * it fixes a collaboration- and public-facing message,

I'm requesting that it be approved for deployment outside of the normal update cycle.