Only allow valid start and end dates for epics

As a User of Epics,

I need to easily manage my Epic's Start and Due dates,

So that I can quickly and simply plan and schedule work.

Now that Epics can inherit the start/due dates of their children (#7332 (closed)) we need to ensure that an Epic can only have valid Fixed start/due dates.

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.