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
Added Brandon DeRosier to authors
Datetime localized on gitlog page of sysadmin dashboard
consolidate new post form variants into single client-side template/view
Merge pull request #4050 from mtyaka/linkedin-email

Make LinkedIn auth provider request email by default.
Merge pull request #4210 from bdero/bdero/fix-sysadmin-dashboard-gitlog-timezone

Make sysadmin dashboard gitlog date timezone-aware
Merge pull request #4216 from edx/jsa/new-post-redesign

consolidate new post form variants into single client-side template/view