DEV: Run `yarn install` when running `bin/ember-cli` (PR #13102)

Some people have noticed that if we change the packages in package.json that they have to manually run yarn install or Discourse won’t work.

This adds yarn install to the bin/ember-cli helper we run. It seems quite fast if there is nothing to install so it shouldn’t hurt to do this every time we start the server.

GitHub

This likely needs shell escaping in case of directories with spaces.

I wonder how many developers do that! I think we can wait and see.