Allow dots in user and group names

[psrok1]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

What is the current behaviour?

I noticed this problem during integration with OIDC SSO where dots were used in preferred usernames. We don't have any good reason to ban this character.

What is the new behaviour?

Dots are allowed in user and group names.