Bug 853933 - Don't inline all locales as JSON in the HTML

Currently, in order to ensure that apps are translated before first paint, all l10n resources are included as one multi-locales JSON dictionary, which is serialized in an inline `<script>` element for each HTML document.  This works fine when shipping few locales, but it can increase the document size quite badly for 60+ locales, thus delaying the startup time very significantly.

This patch allows to support 60+ locales more easily by having:
* one minimal JSON dictionary per locale in each HTML file;
* one extensive JSON dictionary per locale in each webapp.

The inline JSON dictionary is split on a per-locale basis (= one `<script>` node per supported locale) and only contains the l10n strings that are required to translate the HTML document: the "startup cost" of each locale becomes neglectable, and it removes the risk of high RAM consumption when parsing a giant multi-locale JSON at startup.

All l10n resources that are not declared in the HTML document are concatenated in a single JSON dictionary per locale: this reduces the number of disk I/O to load l10n resources and does not increase the total disk size of each webapp.  To enable this, this patch implements a `{{locale}}` quasi-litteral in the HTML markup that is used to declare l10n resources:
  <link rel="prefetch" type="application/l10n" href="/{{locale}}.json" />
where `{{locale}}` is replaced by the value of `navigator.language`.

Note that this latter optimization is only interesting for apps that wait for the `localized` event to start. With the inline JSON dictionary for the declarative content and the `mozL10n.localize()` method, most apps could probably stop waiting for this `localized` event now.
Merge pull request #10709 from fabi1cazenave/l10nOptim-bug853933

Bug 853933 - Don't inline all locales as JSON in the HTML, r=ochameau
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