Move regular expressions for user name from Account to ExternalId

User names are stored as external IDs, hence the regular expressions to
validate the format of a user name should be part of the ExternalId
class.

There is one place in the GWT UI where user names are validated. Since
the GWT UI doesn't have access to the ExternalId class it needs to copy
two of the regular expressions. This should be OK since the format of
user names is not expected to change and the GWT UI will be gone soon.
It's actually a goal to remove all dependencies from the GWT UI on the
Account class so that the Account class can be moved out of the
reviewdb.client package and be turned into an immutable AutoValue type.

Change-Id: I6ea7139019eb0e7f0e06efaf77ec4bd89d514006
Signed-off-by: Edwin Kempin <ekempin@google.com>

gerrit-server/src/main/java/com/google/gerrit/server/account/CreateAccount.java

@@ -111,7 +111,7 @@ public class CreateAccount implements RestModifyView<TopLevelResource, AccountIn
       throw new BadRequestException("username must match URL");
     }
 
-    if (!username.matches(Account.USER_NAME_PATTERN)) {
+    if (!username.matches(ExternalId.USER_NAME_PATTERN_REGEX)) {
       throw new BadRequestException(
           "Username '" + username + "' must contain only letters, numbers, _, - or .");
     }