Add support for creating a user login containing a dot

Dots in user names currently break (possibly among other things) notification preferences. Code that needs to be fixed (this list doesn't intend to be complete):

1. https://github.com/xwiki/xwiki-platform/blob/0bd5585f5b7b0045ef197d26f3c5b69a25ee42bf/xwiki-platform-core/xwiki-platform-notifications/xwiki-platform-notifications-ui/src/main/resources/XWiki/Notifications/Code/NotificationPreferenceService.xml#L41-L47 (saving of notification preferences, treats everything with a dot as document reference)
2. https://github.com/xwiki/xwiki-platform/blob/cebf9167e4fd64a8777781fc56461e9abbe0b32a/xwiki-platform-core/xwiki-platform-notifications/xwiki-platform-notifications-ui/src/main/resources/XWiki/Notifications/Code/NotificationsFiltersPreferencesMacro.xml#L1104 (missing escaping in the LiveTable)
3. https://github.com/xwiki/xwiki-platform/blob/a00ed1be3606d2e8abcc7b574c11d159fc674182/xwiki-platform-core/xwiki-platform-notifications/xwiki-platform-notifications-sources/src/main/java/org/xwiki/notifications/sources/internal/UsersParameterHandler.java#L81 (user parameter handler for notifications, also treats everything with a dot as document reference)

With external authentication like OIDC, it is possible to create such user names (but as far as we're aware, all authenticators are currently normally removing dots by default).