This is how `loadScript(url)` currently deals with multiple concurrent requests
1. Check existing `<script>` tags, and mark existing scripts (other than the
input `url`) as loaded
2. Find "true" `url` of the requested resource (CDN, subfolder path, etc)
3. Check if we have loaded the resource with that "true" `url`, and resolve
immediately if we have
4. Otherwise insert a `<script>` tag with the "true" `url` to load it
For example, in a subfolder install:
- Input `url` = `/javascripts/script.js`
- "True" `url` = `/subfolder/javascript/script.js`
And the _very_ subtle bug here is that we should use also use the true `url`
for step (1), because:
- Since the input and true `url` are different, we mistakenly mark the true
`url` as loaded in step one
- After finding the true `url`, and setting `loaded[trueUrl] = true` in (1), we
resolve the promise prematurely, when the resource could still be loading
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