[C1] REST API: add get_user_from_username

[ywarnier]

Discussed in #5921

Create a service similar to get_users but that only returns one user based on its username.

[ywarnier]

I went ahead and created it, ready for the release of 1.11.30

[ywarnier]

Just git cherry-picking the commit referenced above should be enough to add this service to your 1.11.28, though (just in case). Or you could just insert the few lines manually.

[MeInTheGalaxie]

@ywarnier

Thank you very much, I will try this soon !!! :)

[MeInTheGalaxie]

Thank you Yannick, it works perfectly.