Improve accessibility of issuables list

I have done a screen recording of a issuables list which shows that information like Milestones and Labels are not friendly for screen readers:

https://youtu.be/T4i8OF9Vi4U

As you can clearly see, the focusable buttons on the bootstrap page works fine and the aria-describedby is read later on.

Our tooltips do not open on focus (Milestone Link and Deliverable Label) and are therefore not read, I had to hover manually to get the tooltip read by voiceover.

Proposal

We should improve the a11y of the issuables lists. It is an entry point for every user. Good things to start would be providing better aria-describedby to links, especially links to Issues, MR, Epics and so forth.

Links / references

Related issues:

This discussion from gitlab-ee!4935 may provide some reference.