Introduce user centric tooltips

Problem

Currently when you hover over a username mentioned in a conversation/readme or commit, the tooltip just shows that person's name. Human's can work with text, but are even better at recognising pictures.

After the introduction of:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14377
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14380

we can take one more step to make user tooltips more human centred, similar to slack

Proposal

Include the user's avatar and username, alongside their full name in the tooltip to make the tooltip rich and informative.

Design

• Tooltip with avatar is only necessary if there is only a name or username, so without a companion avatar already next to it. For example, on the issuable sidebar and within comments, the name and username is right next to the avatar so it is unnecessary to display a tooltip as well.