[fix bug 979837] Add Mozilla Webmaker to External Accounts
Merge pull request #851 from mishravikas/fix_979837

[fix bug 979837] Add webmaker to External Accounts
[fix bug 930999] Add username_not_url validator to external accounts.

Refactor username_not_url validator

Simplify username validator

Change _lazy to _

Simplify username_not_url
[bug 930999] Add ExternalAccountForm tests.