Converted hacktionary-data to requirejs i18n bundles.

For more information on i18n bundles, see:

http://requirejs.org/docs/api.html#i18n
converted MDN_URLS to requirejs i18n bundles.
Internationalized slowparse-errors.

This is sort of weird and inefficient right now because we convert
from HTML to an i18nbundle and back again. However, in production
builds, we will be replacing the 'nls' paths with pre-built bundles
anyways, so only one direction will be computed in production.

Ideally we might want to move all this i18n stuff into slowparse,
though, and just couple slowparse to requirejs.
moved content in js/fc/nls/root/ into parent dir.
added build-i18n.js, which makes a big JSON blob of all bundles.
added custom metadata to some i18n bundles.
in build-i18n.js, add selective module export and allow underscore importing.
Added a very simple inline-l10n module.
added js/fc/nls/templates.js and modified template plugin to also localize.
internationalized all templates.
added misc i18n bundle and made friendlycode.js use it.
made fc/ui/publish.js use fc/nls/misc.
internationalized social-media.js.
added some metadata to i18n bundles.
renamed fc/nls/templates to fc/nls/ui, and merged fc/nls/misc into it.
added descriptions to slowparse error i18n bundles.
for i18n bundle keys that originate in templates, show the template filename as part of the key's help metadata.
documented inline-l10n.js.
force requirement of requirejs 2.1.x in package.json.
Add error output for requirejs optimization failure.

Also see https://github.com/jrburke/requirejs/pull/568.