Issue board swimlanes

Description

When the number of open issues grows, adding another sorting view becomes necessary and swimlanes are a common solution. Being able to group issues by Priority or Milestone or User or Team (label) and show them in their workflow stage is a very common practice when the number of issues flows off the screen.

Proposal

Add another dimension to the issue boards so in addition to vertical lists, there are horizontal swimlanes. Swimlanes can divide issues up by:

Priority
Milestone
User
or specific labels

The existence of swimlanes may allow more complex issue board types (https://gitlab.com/gitlab-org/gitlab-ee/issues/977) or may simply be an orthogonal feature to layer on existing types.

Links / references

Suggestion by @dhekimian in https://gitlab.com/gitlab-org/gitlab-ce/issues/17907#note_12940204
Builds on https://gitlab.com/gitlab-org/gitlab-ee/issues/977

/cc @JobV