Update labels in gitlab-ui to use interactive popover
gitlab-ce~9594407 since GitLabbers use labels heavily.
---
### Problem to solve
We currently have links included in label tooltips, which are not particularly helpful as tooltips disappear as soon as you hover outside the element.

### Design Solution
| Labels | New label form |
| ------ | ------ |
|  |  |
<details>
| New label form | Labels |
| ------ | ------ |
|  |  |
| Links should be blue, scoped labels `?` should link to docs | Convert description `input` to `textarea`; add markdown help text |
<summary>Previous design</summary>
From @cperessini's comment in https://gitlab.com/gitlab-org/gitlab-design/issues/282#note_150466105

</details>
**Blocked by:** https://gitlab.com/gitlab-org/gitlab-ui/merge_requests/855
issue