Create a "stable" URL

[abettermap]

Description

We currently have three environments:

1. Prod
2. Local dev
3. Deployed review/staging

Item 3 is very ephemeral, and Item 1 is using production backend, firebase, tokens, etc. Would be nice to have a permanent stable branch and URL for testing.

Resolution

1. Set up GH Actions so that any commit on master should trigger a push (or merge) into a dev or stable branch. This should already trigger a GH Actions flow to the gh-pages branch for any branch except master.
2. Add URL to README. Should be something like https://covid-self-report-tool.github.io/cov-self-report-frontend/review/stable (see deployed gh-pages dirs here).