CLAM accepts Github IDs with invalid characters

[zdw]

Describe the bug

A user can accidentally add an invalid GitHub ID, such as an email address (with . and @ characters) accidentally.

Per https://github.com/shinnn/github-username-regex the signup page only allows alphanumeric data and hyphens (not at beginning or end), so github ID's should have at least a minimal checked for validity when entered.

To Reproduce

1. Enter an invalid github ID for a user
2. CLAM accepts it

Expected behavior

Invalid GitHub ID's are not accepted.

Additional context

User hit the wrong button, entered email address as github ID, and it was accepted, leading to confusion about why CLAM was still rejecting patchsets when the email address was registered.