Injects environment variables into the app through webpack, rather than
script snippets in index.html.
As part of this:
* Updated server.js to use node’s path package to resolve file paths
* Ensure that all url environment variables passed to the app don’t
have a trailing slash, allowing for cleaner template urls
**Note**: There are still a number of constants that should be taken
out of the app and put into environment variables.
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