Enable users to mark Issues and Epics as dependencies for other Issues, and Epics

This Issue is being discussed here https://gitlab.com/gitlab-org/gitlab-ee/issues/2035 as part of this epic &316 (closed)

As a user of issues and epics,

I need to be able to map out dependencies between different issues and epics,

So that I can plan my effort properly through a solid understanding of what sequence the work needs to happen in and what can be down in parallel.

Currently we allow users to mark issues as "Related to.." another issue:

While this is helpful, it doesn't inform me HOW the item is related. Is it related because it's in the same section of code? because it is similar work to effort happening on another section of the product? etc...

To properly enable users to map out dependencies and effort sequencing, they need to be able to mark items as dependent on another item, or that an item is blocking another item of work.

Proposal: We add 2 additional areas to the issue and epic view, in the same area as "Related to:"

• Blocks...
• Is Blocked by...

(The text used should be discussed, as we have other options like "Depends on..." "Is a Dependent of..." etc)

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