ErrNameCharsNotAllowed is raised even though username is possible

[DasRoteSkelett]

• Gitea version (or commit ref): master (0903b1a)
• Git version: 2.25.1
• Operating system: Ubuntu 20.04
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
  • Not relevant
• Log gist:
  2020/05/20 12:46:17 ...dels/login_source.go:804:UserSignIn() [W] Failed to login 'matthias.schöpfer' via 'ithinx ldap': User name is invalid [Matthias.Schöpfer]: must be valid alpha or numeric or dash(-_) or dot characters

Description

One of my former, brain dead administrators gave me a username containing a special german letter. Now, ldap will not function because my user name gets rejected by gitea.

To my understanding, in user.go, line 92 / 93 must be patched for that to work:
// Characters prohibited in a user name (anything except A-Za-z0-9_.-) alphaDashDotPattern = regexp.MustCompile(``[^\w-\.]``)
I do not know how go works, so this will probably not be a big deal I guess?!

[lafriks]

we support only alphanumeric characters. We could probably add support for ASCII transliteration so that as your example matthias.schöpfer would be transformed into matthias.schopfer that would be valid

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