Add milestone, my-reaction and confidential tokens to issues refactor

What does this MR do?

[This MR adds milestone, my-reaction, and confidential filtered search tokens to the issues list page refactor, which is under the feature flag vue_issues_list defaulted to off.

List of issues list page refactor steps:

• Add missing UI elements to issue list card — Done in !55699 (merged)
• Add pagination — Done in !55699 (merged)
• Add sorting — Done in !56754 (merged)
• Add bulk edit — Done in !57230 (merged)
• Add tabs and header buttons — Done in !57549 (merged)
• Add searching and empty states — Done in !58774 (merged)
• Add author, assignee, and label filter tokens - Done in !59490 (merged)
• Add milestone, my-reaction, and confidential filter tokens — This MR!
• Add the other missing filter tokens

Issue: #322755 (closed)]

Screenshots (strongly suggested)

Screen_Recording_2021-04-23_at_12.57.22_pm

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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

Related to #322755 (closed)