Fix EpicToken rendering and filtering

What does this MR do?

This MR fixes following issues with epic_token.vue used for Vue Epics List and Vue Issues List (both features currently behind feature flag).

• Fixes an issue in Vue Epics List where rendering list of values for token Epic leads to duplicate keys console error when you have epics from child groups.
• Fixes a bug where epic from child group fails to fetch when you select it from the dropdown list for Epic token.

Screenshots (strongly suggested)

This MR doesn't change anything visually as both are bugs that happen behind the scenes.

Does this MR meet the acceptance criteria?

Conformity

• I changelog entry is not needed, as token usage is only present for features currently behind the flag not enabled by default. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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