Change default backend config setting for EMAIL_CONTACT_ADDRESS to dummy variable

[dakotabenjamin]

ref

tasking-manager/backend/config.py

Line 60 in 8ee05dc

"TM_EMAIL_CONTACT_ADDRESS", "sysadmin@hotosm.org"

Orgs that have been setting up their own versions of TM have been leaving this variable blank, so our sysadmin email continues to get their messages.

We should ensure that when changing the variable, that our own instance of TM has the var set properly and not fallback to the default.

[linear]

TECH-807