Add Asana integration

**This is a work in progress.** And I created this PR early to let you know that I'm working on an Asana integration.

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**What does this MR do?**

When someone commit with a message including

 - "blabla #123456", it will attach a new message to the task 123456 in Asana.
 - "fix #456789", it will attach a new message and close (mark as done) the task 456789 in Asana

**Are there points in the code the reviewer needs to double check?**

I guess `app/models/project_services/asana_service.rb`

**Why was this MR needed?**

We use Asana as our Issue tracking system. It will ease the relation between Asana and Gitlab by linking Asana tickets to git commit.

**What are the relevant issue numbers / Feature requests?**

Nope.

**Screenshots (If appropiate)**

WIP :wink: 
Prefer single-quoted strings when you don't need string interpolation or special symbols.