Add allow_sign_up setting for auth.google/github.

Fixed issue with unsaved changes srv being triggerd by starring / unstarring a dashboard, Fixes #1795

oauth google: read api_url from config file

Data source proxy: Fixed issue with using data source proxy when grafana is behind nginx suburl, Fixes #1784

Graph Panel: Legend in table mode now aligns, graph area is reduced depending on how many series, Fixes #1786, Fixes #1749

Support for unicode / international characters in dashboard title (improved slugify), Fixes #1734, Fixes #827

instead of using https://www.googleapis.com/oauth2/v1/userinfo in
code.

sync from upstream

#1781 try fix oauth with github enterprise - add config of api_url

read github api from config file instead of using public github:
http://api.github.com/user

this would be useful when you are using github enterprise edition