Reduce size of `index.html`

[slifty]

Copying the body of the related JIRA ticket https://permanent.atlassian.net/browse/PER-9983:

Our index.html files are pretty big and hard to manage and keep in sync with each other. This is mainly because of some environment-specific script files embedded into each. Figure out if there’s a way to hide these scripts away so the index.html files are smaller and easier to deal with.

[slifty]

While working on this I want to flag that this issue is related to a symptom, I think, rather than the root cause.

There are two issues here:

1. We have a separate index.html for each environment, as opposed to using Angular's build tooling to have a single index which is generated with the appropriate environment inserted.
2. We have hard coded vendor scripts instead of installing them as a package and injecting them via Angular.

I have a commit to address the original issue (moving common code to a single file), but what I actually would like to do is address (1) and (2) above.

I won't make a new issue to capture that work, since I think this is still ultimately a solution that addresses the issue!