Create User fails

[tbsbdr]

Steps to reproduce

1. Try to create new user: Admin settings > Users > New User
2. Enter Hänschen as user name, click Create button
3. Error shows Failed to create user

Expected behaviour

• Add validation to the user name field: Tell the user which characters are allowed
• "Create" can only be clicked if the username is valid

Actual behaviour

• Create User fails

[tbsbdr]

cc @nicholas-wilson-au

[JammingBen]

@micbar Could you provide us a list of characters that are not allowed in a username?

Ideally, the server would tell the client all the allowed characters, but for file names we don't have this either. Should be fine to hard-code such rules in Web (for now at least).

[tbsbdr]

Hardcoding is fine for now I'd say 👍 we don't need to over-perform here.
Pointer where allowed characters are defined @JammingBen:
https://github.com/owncloud/ocis/blob/8cbf261568197a5494931357b5318d57b7134ae9/services/graph/pkg/service/v0/users.go#L742