Add spinner to Roadmap

What does this MR do?

This MR adds a spinner to the Epics Roadmap since there was no indication before that anything was being loaded

Issue: #215077 (closed)

Screenshots

Screen_Recording_2020-05-05_at_12.27.54_pm

The video shows a roadmap loading for 1) a group that has epics, and 2) a group that has no epics

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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