Add epic filter token to issues page refactor

What does this MR do?

This MR adds the epic filter token to the issues page refactor, which is under the feature flag vue_issues_list defaulted to off.

epic_token.vue was also changed to be more in line with the other filtered search tokens (such as author_token.vue):

• Before, it received a value that was either an epic iid or a string in the format <epic.title><epic.iid>, and received two separate functions for getting epics
• After, it was simplified to receive only a epic id/iid, and receive only one function for getting epics.
• These changes only affect the roadmap (where epic_token.vue is currently used) and was confirmed to be working after these changes.

This MR creates iteration and weight filtered search tokens, and adds them 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 — 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 epic filter token — This MR!
• Add None/Any options to search tokens
• Add release token

Issue: #322755 (closed)

Screenshots (strongly suggested)

Screen_Recording_2021-05-05_at_6.41.54_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)