Add opt-out functionality to all secure signal sample pages

[eiman-eltigani-ttd]

1. .env variables for testing client-side (sourced from uid2-operator repo in resources/test/client_side_keypairs/client_side_keypairs.json):

UID2_BASE_URL="http://localhost:8080"
UID2_CSTG_SERVER_PUBLIC_KEY="UID2-X-L-MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEtXJdTSZAYHvoRDWiehMHoWF1BNPuqLs5w2ZHiAZ1IJc7O4/z0ojPTB0V+KYX/wxQK0hxx6kxCvHj335eI/ZQsQ=="
UID2_CSTG_SUBSCRIPTION_ID="4WvryDGbR5"

2. .env variables from testing server/client-server.js (sourced from clients/clients.json):

UID2_BASE_URL="http://host.docker.internal:8080"
UID2_API_KEY="UID2-C-L-124-H8VwqX.l2G4TCuUWYAqdqkeG/UqtFoPEoXirKn4kHWxc="
UID2_CLIENT_SECRET="NcMgi6Y8C80SlxvV7pYlfcvEIo+2b0508tYQ3pKK8HM="
SESSION_KEY="any-random-string-here" #this is needed for server-side, it can be any string

[eiman-eltigani-ttd]

Implementation details:

• For server/client-server, opt-out status is returned as response.status === 'optout' from the /token/generate endpoint, so I added an additional check in server.js to render a newly created optout.html page
• Client-side wasn't as straight forward because the UID2 SDk returns opt-out as {identity: null} which is the same when user hasn't generated any token so I had to create a loginAttempted flag to differentiate optout

[eiman-eltigani-ttd]

Also created an optout banner because the response for opt-out and initial page load is the same.

[eiman-eltigani-ttd]

The fetch/AJAX request was intercepting normal form submission and wasnt allowing other pages to render properly (now the error page and optout page are displaying as expected). Also secure signals are loaded/cleared automatically based on the identity state so it was simpler to remove it all

[eiman-eltigani-ttd]

Lastly, I applied the same logic I used on the client side after getting that working and tested properly. I still need to verify that it works locally after the Dockerfile is setup correctly/any final changes are made.

[ssundahlTTD]

I think this may be breaking the non-optout flow. It's not working anymore.

[eiman-eltigani-ttd]

Updated env.sample file and added SESSION_KEY so users know thats needed to run server-side site

[eiman-eltigani-ttd]

Was able to get React running with npm start by downloading dotenv-cli which allows you to load environment variables from a specific file path, updated the instructions in the ReadMe and env.sample file as well