Use network name in email subjects without baking in "Codidact"

[cellio]

#1663 made subject lines on email we send less generic, because a lot of "Confirmation instructions" email was getting marked as spam and I thought making it more specific would help avert the automatic reaction. However, that made the problem of hard-coding the string "Codidact" worse.

The subject lines are in a strings YAML file, and I don't know of a way to make those dynamic. We need to have the code that sends the email construct a subject line from ENV.fetch('NETWORK_NAME', 'Codidact') and a generic string (and make the strings generic again).