inline AuthorInput.getHintRangeFromCursor to address TypeError

[shiftkey]

Fixes #4847 by inlining the function.

Before:

• run current master with yarn build:prod and yarn start:prod
• choose a GitHub repository
• try and add a co-author
• 💥 no auto-complete appears, errors in dev tools console

Which is this unminified code in onAutocompleteUser:

After:

• run this version with yarn build:prod and yarn start:prod
• choose a GitHub repository
• try and add a co-author
• ✅ auto-complete appears, filtered users listed, no errors in dev tools console

This is the new unminified code for onAutocompleteUser:

Other notes:

I could reproduce this with the latest webpack (4.10.2 as of writing) but I think this needs to be isolated and submitted to uglify-es because that's the component doing the JS mangling here.

[niik]

This is highly disturbing if we can't trust the magnification step any more. Let's get this workaround in but we should definitely try to get to the bottom of what's causing this.