gitlab support added

[kounelisagis]

We now have GitLab support!

Major changes:

• Dockerfile, Dockerfile-dev - added instruction according to this to solve the dep ensure problem described.
• Gopkg.toml - added go-gitlab client library for gitlab api support. docker-compose.yml, dev.docker-compose.yml - added environmental variable for gitlab token
• Project Model - 1) new field named "is_github" was added in order to determine if the project is github or gitlab. 2) Field named "github_id" was changed to the more generic "issue_id".
• worker2/worker2.go - the new gitlab worker works similar to the github worker.

[alexdor]

@kounelisagis you have multiple conflicts on this merge request

[alexdor]

@kounelisagis this pull request has been reverted because it wasn't approved and it was breaking the backend, it would be amazing if you could fix your pull request and re-open it so it can actually get merged in the project, don't hesitate to contact me if you need any help