Make location of external js dependencies configurable [$15]

[acooks]

To comply with Fedora packaging guidelines, dependencies like d3.js need to be installed system-wide, away from the rest of the web assets.

However, some use cases require the js dependencies to be served by the jittertrap server, like running on an isolated test network. (As it works currently)

To do:
Review html5-client/src/html/index.tpl.html and make the location of each of the 3rd-party javascript dependencies configurable, so that it's possible to comply with Fedora packaging guidelines. A simple compile-time 'sed' or 'perl' template replacement in html5-client/Makefile may be sufficient.

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