Allow pausing of initial messages

[jeffm2001]

Description

We've had instances where we needed to pause all our campaigns but still wanted texters to be able to respond to replies for messages that have already gone out. This makes it possible to do that by setting a DOWNTIME_NO_INITIAL environment variable. When the environment variable is set, instead of seeing the normal texting interface texters will see a message saying "Sending initial messages is currently paused. You can still reply to any incoming messages."

Checklist:

• I have manually tested my changes on desktop and mobile
• The test suite passes locally with my changes
• If my change is a UI change, I have attached a screenshot to the description section of this pull request
• My change is 300 lines of code or less, or has a documented reason in the description why it’s longer
• I have made any necessary changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• My PR is labeled [WIP] if it is in progress

[engelhartrueben]

I really like the idea, but am hesitant with with instances that have many organizations running/texting concurrently. Also, having to restart the application and set DOWNTIME_NO_INITIAL may be too disruptive (again, when there are many orgs).

Has this been mostly used when there has been something wrong with the application? Or to stop texting in general?

[jeffm2001]

The goal was to have something slightly less disruptive than setting DOWNTIME or DOWNTIME_TEXTER. I think we used it twice, once when we were switching from Twilio to Bandwidth and needed to wind down active campaigns, and once when we were dealing with scaling/instability issues and wanted to scale back activity without shutting down entirely.

Agree that it would be nice if it could be done on a per-organization or even per-campaign basis, but the implementation of that would get trickier.

[engelhartrueben]

🎉