Bug 41698: Reword the recommendation badges in about:addons

Firefox strings use { -brand-product-name }.
As a result, it seems that the fork is recommending extensions, whereas
AMO curators are doing that.
So, we replace the strings with custom ones that clarify that Mozilla is
recommending them.

We assign the strings with JS because our translation backend does not
support Fluent attributes, yet, but once it does, we should switch to
them, instead.

Upstream bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1825033