[Feature Proposal] Display name instead of / besides username for @mentions
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### Description
`@mentions` now show up as `@<username>` in tickets, MR's, etc. When you hover them the user's `Name` is displayed. We run Gitlab in an on-prem enterprise environment connected to Active Directory via LDAP, so our user accounts in Gitlab look like this:
- Name: Stefan van Gastel
- Username: u00ab12
- Email: s.v.gastel@example.com
In your typical issue or MR discussion this results in conversations like this:
```
Love this feature, though @u00ab12 might want to take a look at the test coverage before @u00xy89 can merge it.
```
This requires us to either recognize our colleagues account names or hover every `@mention` to display the name and get the full context of the messsage.
### Proposal
To display the users `name` instead of / as a postfix in @mentions like this:
```
Love this feature, though @Stefan van Gastel might want to take a look at the test coverage before @John Doe can merge it.
```
or
```
Love this feature, though @u00ab12 (Stefan van Gastel) might want to take a look at the test coverage before @u00xy89 (John Doe) can merge it.
```
If the way of displaying could be configured as a personal of global Gitlab setting it would not change anything to existing behaviour, only improve it for situations where non descriptive usernames are used.
#### Use cases
Improves overall readability (and thus productivity) of issues, comments, merge requests, etc in cases where non descriptive usernames are used.
### Feature checklist
Make sure these are completed before closing the issue,
with a link to the relevant commit.
- [ ] [Feature assurance](https://about.gitlab.com/handbook/product/#feature-assurance)
- [ ] Documentation
- [ ] Added to [features.yml](https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/data/features.yml)
issue