Add GitLab support

[bacharakis]

• Research, identify and select a client library for fetching GitLab issue.
• Implement code changes
• Review and testing

[kounelisagis]

Update - July 1st

We already have a client library to send requests to the GitHub API using GraphQL
(https://github.com/shurcooL/githubv4). So we need something similar for GitLab API. Unfortunately there is no such library for GitLab. Therefore we are going to use the classic GitLab RESTful API (https://docs.gitlab.com/ee/api/README.html).

Future Updates

When GitLab GraphQL API becomes available we can replace the specialized version of the client library, targeting only GitHub, with the universal version: https://github.com/shurcooL/graphql. In this way we can call both APIs in a similar way using just one universal GraphQL library. Furthermore, the current source code is expandable because the new library is just the generalization of the old. Only few changes are needed.

[kounelisagis]

Update - July 12

The changes can be viewed here: https://github.com/kounelisagis/fixme_backend/tree/gitlab