Enable GitHub repos as --template targets

Created by: JasonEtco

👋 This should enable users to run:

npx create-react-app target --template owner/repo

All I did was make the starting @ optional in the scoped template string check, before prepending cra-template-. I don't believe this will break scoped templates, I had a hard time getting the tests to run locally.

Unfortunately, this doesn't work at the moment - it looks like CRA expects to be able to use the template string directly in a require, so it tries to do require('owner/repo'), when it should be require('repo'). That looks like a larger change though 😅 would love some help figuring out the best way to pass that along (cc @iansu)!

Here's the error output I'm seeing at the moment:

Error: Command failed: node /Users/jasonetco/dev/create-react-app/packages/create-react-app/index.js my-app --template iansu/cra-template-wheeler-mode --scripts-version="/Users/jasonetco/dev/create-react-app/packages/react-scripts/react-scripts-3.3.0.tgz"
    at checkExecSyncError (child_process.js:621:11)
    at Object.execSync (child_process.js:657:15)
    at Object.<anonymous> (/Users/jasonetco/dev/create-react-app/tasks/cra.js:115:4)
    at Module._compile (internal/modules/cjs/loader.js:956:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:973:10)
    at Module.load (internal/modules/cjs/loader.js:812:32)
    at Function.Module._load (internal/modules/cjs/loader.js:724:14)
    at Function.Module.runMain (internal/modules/cjs/loader.js:1025:10)
    at internal/main/run_main_module.js:17:11 {

Closes #8166