Add ':' to $wgInvalidUsernameCharacters
This will prevent the creation of new users with a colon character
in their username. Existing accounts or system users will not be affected.
Colons themselves are not an issue, however they are used as the delimiter
for namespaces and interwikis. In a global account type system where there
are wikis with multiple languages, usernames that begin with a localized
namespace prefix will be invalid on some wikis and not others, which is
problematic. Preventing the usage of colons avoids that issue entirely.