Bug 773371 - [bedrock] Integrate Language Selector for M.o

Implemented a simple language switcher with a list of all available
translations of the current page. The `translations` variable can be
used for other purposes like a universal language selector or
`rel="alternate" hreflang="x"` (Bug 875293).
Implemented canonical and alternate URLs
Added canonical URLs to the non-responsive template
Bug 834422 - Add Open Graph Protocol (OGP) and Twitter Cards

Also fixed Bug 850164 - [SEO][Bedrock] meta descriptions are missing,
or shared by several pages