Limit number of shown environments

What does this MR do?

This MR limits in context of Merge Request a list of shown environments. Previously we would show all environments containing the SHA of the head commit of Merge Request. However, with introducing of dynamically created environments this lead to a cases that we would show multiple review apps, for different branches, because these branches would contain a new questioned commit.

This MR changes what environments we test against presence of the commit, to:

1. We look for environments with deployments to source_branch of source_project: used for deployments to per-branch environments,
2. We look for environments with deployments to target_branch of target_project: used for deployments to staging / production environments,
3. We look for environments with deployments for tags on target_project: used for staging / production environments.

Why was this MR needed?

To improve a list of returned environments when we introduced ability to create dynamic environments for review apps: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6323

Screenshots (if relevant)

This allows us to limit this list:

To:

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

None yet, but they would be created when users would start using review apps.