Add iteration and weight filter token to issues list refactor

What does this MR do?

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 iteration and weight filter tokens — This MR!
• Add the other missing filter tokens

Issue: #322755 (closed)

Screenshots (strongly suggested)

iteration	weight
Screen_Recording_2021-04-23_at_5.00.20_pm	Screen_Recording_2021-04-23_at_6.25.58_pm

Does this MR meet the acceptance criteria?

Conformity

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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Security

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