Inject server env vars into index.html, including instance id (#2888)

## Description:

Should fix the broken 1v1 on staging. The issue was that we had multiple
staging environments, and the matchmaker would often route a player to a
game on a different staging server, so the client couldn't find the
game.

So now each deployment has a unique id, and the matchmaker only connects
players & servers that have the same instance id.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

evan

package.json

@@ -104,6 +104,7 @@
     "compression": "^1.8.1",
     "dompurify": "^3.1.7",
     "dotenv": "^16.5.0",
+    "ejs": "^3.1.10",
     "express": "^4.22.1",
     "express-rate-limit": "^7.5.0",
     "fastpriorityqueue": "^0.7.5",