User flair was given by user's primary group. This PR separates the
two, adds a new field to the user model for flair group ID and users
can select their flair from user preferences now.
This can happen when an avatar-flair component is rendered to an anonymous user on a login_required site (e.g. when they are redeeming an invite). The lack of group information was causing an error to be raised. With this commit, it now simple skips rendering the flair.
Rendering an empty flair element with the css `background-image: url();` causes the browser to attempt an image request against the current document URL. Making duplicate requests for the document URL can cause some unusual race conditions, especially related to cookies. If this user-avatar-flair element was present on the site homepage (e.g. if categories+latest is the homepage), then it can prevent the signup flow from working correctly.
This commit updates the user-avatar-flair component to be a transparent wrapper around the avatar-flair component. If the user has no flair, no avatar-flair element will be rendered. This avoids the `background-image: url();` situation, and fixes the auth flow.
This commit also removes the duplicate avatar flair rendering from the `latest-topic-list-item` component. This wasn't particularly obvious, since the duplicate flairs were being rendered directly on top of each other.
