store_l10n.py: Fetch strings from a GitHub mirror and l10n.mozilla-community.org

[JohanLorenzo]

This issue should have been filed at the same time as #36 and #37

Bug 1383642 caused 55.0b12 to not be uploaded. This was a little bit silly because almost no strings had to be updated on Google Play.

There are several ways to solve that problem (see #36, #37 and mozilla-releng/pushapkscript#25). The easiest one remains the following:

Instead of contacting l10n.mozilla-community.org here, we can just have 2 places to fetch strings from. One is the regular service (l10n.mozilla-community.org), the other may be a GitHub repo, which mirrors l10n.m-c.o. Thus, if one fails, the other can be contacted.

Mirroring the GitHub repo will probably require a bumper bot, like the one used by Bedrock or the L10n bumper bot in mozilla-beta/mozilla-central.

cc @MihaiTabara @flodolo

[flodolo]

(also cc @Pike)