Add scope/field selector settings for LinkedIn provider.

This change adds three new configuration variables to third_party_auth's LinkedIn provider:

* SOCIAL_AUTH_LINKEDIN_OAUTH2_SCOPE,
* SOCIAL_AUTH_LINKEDIN_OAUTH2_FIELD_SELECTORS,
* SOCIAL_AUTH_LINKEDIN_OAUTH2_EXTRA_DATA

Being able to configure these additional settings is useful if you want
the LinkedIn provider to pre-populate the email field when a new user
registers via the linkedin provider.

The Google provider prepoulates the email field by default, but if you
want LinkedIn to do the same, these two settings should be set to:

SOCIAL_AUTH_LINKEDIN_OAUTH2_SCOPE = ['r_basicprofile', 'r_emailaddress']
SOCIAL_AUTH_LINKEDIN_OAUTH2_FIELD_SELECTORS = ['email-address']

For more info see:
http://psa.matiasaguirre.net/docs/backends/linkedin.html
Fix saving error of video editor in non-english translation

Currently the code in ```tabs-aggregator.coffee``` uses the display name of a tab as the key to index the current tab. When a non-english translation is applied, the javascript code will get localized names instead of the original english names, which causes the following ```getValue``` function fails as it cannot reference the correct tab.
Use the 'data' attribute to store the original tab name.
Do not look for country if no new ip.
Added Brandon DeRosier to authors
Datetime localized on gitlog page of sysadmin dashboard
Fix modulestore merge conflict with heartbeat.
Corrected event name in description
Merge pull request #4235 from edx/ahodges/documentation/DOC639

Corrected event name in description
ModulestoreEnum class.
Merge pull request #4233 from edx/alex/geoip-none-ip-fix

Do not look for country if no new ip.
Adds is_xblock_visible_to_students
PR 2571, log stop_video event
Merge pull request #4242 from edx/ahodges/documentation/DOC367

PR 2571, log stop_video event
Updated for PR 4106, BLD-1134
Merge pull request #4243 from edx/ahodges/documentation/DOC657

Updated for PR 4106, BLD-1134
Merge pull request #4239 from edx/release

Release
Fix branch settings.
Merge pull request #4236 from edx/nimisha/fix_ms_merge_conflict_heartbeat

Fix modulestore merge conflict with heartbeat.
Merge pull request #4217 from louyihua/master

Fix saving error of video editor with non-english translations
consolidate new post form variants into single client-side template/view