Resolve "Blocking issues MVC: (add support for issue dependencies)"

What does this MR do?

This MR changes the "Related issues" card in the issue page to a "Linked issues" card that allows users to add and view issues that the current issue "relates to", "blocks" or "is blocked by".

This feature is activated using the feature flag :issue_link_types:

bin/rails console
Feature.enable(:issue_link_types)
exit

Issue: #2035 (closed)

Screenshots

Before:

After:

Screen recording of adding and deleting issues:

Screen_Recording_2019-12-18_at_4.22.04_pm

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team