Refactor sidebar weight block to use Vue and be async + add to Issue Boards

What does this MR do?

CE backport, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15706

• Refactor sidebar weight to use Vue and be async
• Add Weight block to Issue Boards

.	Boards

Are there points in the code the reviewer needs to double check?

• Created some EE versions of the mediator and store for weight manipulation
• Usage of glDropdown in Vue which we are already doing in the board version of assignees but this decouples us away from the event hell with right_sidebar.js and issuable_context.js and the usage is pretty programmatic. I used v-once on the .dropdown-content so glDropdown could handle all the manipulation there.

Other notes:

• We can remove def weight_dropdown_tag and weight_select.js(and spec) when the <namespace>/<project>/issues/<iid>/edit endpoint is gone 🤹

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
  • I don't see any existing docs for editing issues in the sidebar so it probably isn't necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes #1379 (closed)