cli, sql, ui: allow all-numeric usernames

[rolandcrosby]

Is your feature request related to a problem? Please describe.
A company evaluating CockroachDB for enterprise deployment ran into an issue in setting up GSSAPI with their Active Directory environment. The company uses numeric employee IDs as Active Directory usernames. CockroachDB requires usernames to start with a letter or underscore, so the company's usernames cannot be mapped to CockroachDB users.

Describe the solution you'd like
The username validation rules should be relaxed to allow all-numeric usernames.

Describe alternatives you've considered
We could also solve this problem by adding more sophisticated functionality for mapping GSSAPI users to CockroachDB users, like Postgres's map functionality, but that would take much more work.

Additional context
Postgres allows numeric usernames:

postgres=# create user "12345";
CREATE ROLE
postgres=# \du
                                   List of roles
 Role name |                         Attributes                         | Member of
-----------+------------------------------------------------------------+-----------
 12345     |                                                            | {}
 roland    | Superuser, Create role, Create DB, Replication, Bypass RLS | {}
postgres=# select version();
                                                     version
-----------------------------------------------------------------------------------------------------------------
 PostgreSQL 11.4 on x86_64-apple-darwin18.6.0, compiled by Apple LLVM version 10.0.1 (clang-1001.0.46.4), 64-bit
(1 row)