[47]: add generic validation handler; add username validation

[mi4l]

Issue

Adds a generic validation handler for all of our server side validation needs. A field value is passed in first, followed by any functions needed to validation that field. The validation function returns any errors, which are then parsed by an error handler if they exist.

This PR also has all of the username validation handlers.

gatherValidationErrors(
    body.username,
    usernameCharactersValidator,
    usernameLengthValidator,
    usernameTypeValidator
);

/review @JacobPernell @mskalandunas