Add contact sales option for Epics

After a quick consultation with @JobV, this issue was split of from https://gitlab.com/gitlab-org/gitlab-ee/issues/4821

Description

We'd like to identify additional opportunities within the product to include a call to action to contact sales (https://about.gitlab.com/sales/).

Proposal

• CTAs should go to about.gitlab.com/sales/
  • We should be able to identify the feature the user clicked on when viewing the submission in Marketo.

How the banners work

• The banner can be dismissed anytime.
• If a user dismiss the banner, the banner will not show up again.
• "Contact sales" button links to Contact Sales page.
• "Read more" links to the specific feature's documentation.

Design

Upgrade Premium plan to Ultimate plan:

Epics:

• Even if the plan is "Starter", it still shows "Epic" in the issuable sidebar. So "Epic" could be a touch point to upgrade the plan (contact sales).
• When clicking "Upgrade plan" text, it shows the dropdown, and then users are able to contact sales.
• Users are able to dimiss the promotional message forever by clicking Don't show me this again button. After clicking it, the Epic section will hidden.

Links / references