Bug 894248 - Fix broken or dead link in /about/powered-by/

Remove 5 dead links + 1 soon-to-be (Songbird), fixed 5 broken links.
Bug 892696 - Change default language for /newsletter

On /newsletter and footer email forms, if the user's locale is not
for a language that our newsletters support, then instead of
defaulting the language field to 'en', default to having it not
set, to force the user to choose one of the supported languages
when subscribing instead of just clicking through and maybe not
wanting English but getting it anyway.
Bug 893165 - /id/newsletter formatting

The issue was that in the header 'Read all about it in our
<span>newsletter</span>', the <span> and </span> each were causing a
line break. In languages like Indonesian where the word order was
changed, e.g.:

    Baca selengkapnya di <span>nawala</span> kami

this was breaking the header up into three short lines.

It turned out that header spans were being styled in newsletter.less
as "display: block". Getting rid of that solves the problem.
Adjust Russian tabzilla promo font size bug 893329
Merge pull request #1077 from dpoirier/bug-892696-change-newsletter-language-default
Merge pull request #1078 from dpoirier/bug-893165-id-newsletter-header-formatting
Merge pull request #1081 from alexgibson/adjust-russian-tabzilla-promo-text-size-bug-869004

Adjust Russian tabzilla promo font size bug 893329
Move matchMedia/scroll pos checks to desktop only. Bug 894962.
Merge pull request #1084 from jpetto/bug-894962-fxos-viewport-bug

Move matchMedia/scroll pos checks to desktop only. Bug 894962.
Merge pull request #1076 from flodolo/bug894248

Bug 894248 - Fix broken or dead link in /about/powered-by/