Add Atlassian Bamboo service

Created by: dblessing

What does this MR do?

Adds support for Atlassian Bamboo as a CI service.

This also includes documentation in doc/project_services. There are some 'gotchas' when setting up triggers with Bamboo. It doesn't support nice webhooks like GitLab CI.

Are there points in the code the reviewer needs to double check?

I think this will cause a merge conflict with enterprise edition. If so, it will be in the app/views/projects/services/_form.html.haml file and is easily resolvable.

I'm also questioning whether we want line 31 changes in app/views/projects/services/_form.html.haml.

You'll probably notice that I listed all services in the new 'Project Services' documentation page but only wrote docs for Bamboo service. It might be ideal to have docs for all services, but that's outside the scope of this PR. I can probably go back and write some other service docs, or someone else can, too.

Why was this MR needed?

We want to use Bamboo as a CI service and it was simple to add. I'm certain others would find use, also.

What are the relevant issue numbers / Feature requests?

None that I'm aware of.

Screenshots

Services list

Service settings page

Project Services help page

Bamboo help page