Use new gitlab_git gem. It makes CONTRIBUTING.md work

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Add default value for Project#imported boolean

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Default values for web hooks

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
 Merge branch 'gitlab_git_5_6' into 'master'

 Use new gitlab_git gem. It makes CONTRIBUTING.md work
 Merge branch 'pg_fixes' into 'master'

Postgres fixes

For applications migrated from MySQL to PostreSQL it fixes some issues
Fixes #1060