Create related_epic_links table

Create a new table related_epic_links (See discussion #351597 (comment 835799403) for naming options) that matches attributes and functionality with issue_links. We don't have to use strictly the same naming if not possible but there should be still 1-1 mapping of attributes/functionality

issue_links table:

issue_links (
    id integer NOT NULL,
    source_id integer NOT NULL,
    target_id integer NOT NULL,
    created_at timestamp without time zone,
    updated_at timestamp without time zone,
    link_type smallint DEFAULT 0 NOT NULL
)

The new model, like IssueLink should define the link types relates_to, blocks and is_blocked_by, as well as checking for opposite and self relations.

This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.