Validate username format before attempting to flush it to the db

[dstufft]

Validate a valid username prior to trying to save it to the database, using the same logic as is already in the database.

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https://sentry.io/python-software-foundation/warehouse-production/issues/258353802/

IntegrityError: new row for relation "accounts_user" violates check constraint "accounts_user_valid_username"

  File "sqlalchemy/engine/base.py", line 1182, in _execute_context
    context)
  File "sqlalchemy/engine/default.py", line 470, in do_execute
    cursor.execute(statement, parameters)

IntegrityError: (psycopg2.IntegrityError) new row for relation "accounts_user" violates check constraint "accounts_user_valid_username"
(30 additional frame(s) were not displayed)
...
  File "warehouse/raven.py", line 41, in raven_tween
    return handler(request)
  File "warehouse/cache/http.py", line 69, in conditional_http_tween
    response = handler(request)
  File "warehouse/cache/http.py", line 33, in wrapped
    return view(context, request)
  File "warehouse/accounts/views.py", line 226, in register
    form.email.data
  File "warehouse/accounts/services.py", line 152, in create_user
    self.db.flush()

IntegrityError: (psycopg2.IntegrityError) new row for relation "accounts_user" violates check constraint "accounts_user_valid_username"
DETAIL:  Failing row contains