Add social media APIs to allow localizers to easily share their actions in Pontoon

[bugzilla-to-github]

This issue was created automatically by a script.

Bug 1383046

Bug Reporter: @guerojeff CC: @mathjazz

Use case: User spends an hour translating strings for Firefox, wants to share that to their networks as a means of recruitment and evangelism. Social media APIs with stock messages for sharing would enable this user to share what they've done and how others can do it to within their social networks.

An example of this can be found here: https://www.google.com/appserve/mkt/vib/ANyC23KZ46prSPMqmjbkFFP-PKYUSvFYlzYF-4ogSlYjoqUbPpZ067oTymPJiq9sPTA1co-JvqMWq_Pvea-GRJLcnObq