Add None/Any search tokens to issues page refactor

What does this MR do?

This MR adds None/Any options to the following in the issues page search bar:

• Assignee token
• My-Reaction token
• Iteration token

The weight token already shows None/Any tokens. None/Any options were not added to the other tokens because they are not available for filtering in the API yet.

This change is part of the issues page refactor, which is behind the vue_issues_list feature flag 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 — Done in !60120 (merged)
• Add iteration and weight filter tokens — Done in !60148 (merged)
• Add "email issue to project" modal — Done in !60430 (merged)
• Add other missing things — Done in !60648 (merged)
• Add None/Any options for filter tokens — This MR!
• Add epic and release filter tokens

Issue: #322755 (closed)

Screenshots (strongly suggested)

Screen_Recording_2021-05-06_at_5.05.04_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)