Add contact sales option for Epics

What does this MR do?

1. Adds a contact sales option into the Issues sidebar, where epics normally are.
2. Adjust current issue weights promotion to match the new specified behavior

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

• The original UX defined a Contact Sales link, but due to ongoing discussion in https://gitlab.com/gitlab-org/gitlab-ee/issues/5734, I'd rather user the existing partial, as the partial does contain a lot of logic to determine what to show to which user. If we'd just put Contact Sales there, we would end up with a worse situation IMHO.

Screenshots (if relevant)

This is the current behavior of the issue weights:

Below you can see the stills and behavior of the new epics callout and the updated issue weights callout. Everything works, including dismissal.

Collapsed	Expanded	Behaviour
		epics-weight-callout-behavior

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [n/a] Has been reviewed by Database
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• [n/a] 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
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #5710 (closed)