Rename blog prop to url in PostAvatar, deal with optional value

[humphd]

We use blog for the link's href in the PostAvatar:

const PostAvatar = ({ name, img, blog = '' }: AvatarProps) => {
  const classes = useStyles();

  if (img) {
    return <Avatar className={classes.avatar} src={img} />;
  }

  if (name.length > 0) {
    const initials = name
      .split(' ')
      // splitName represents the current value, i represents its index, and arr represent the whole splitted name-word array
      // if the index is 0, means it's the first word in the name
      // if the index+1 equals to the array length, it means the current value is the last word in the name
      // anything rather than first word or last word will not be included in initials
      .map((splitName, i, arr) =>
        i === 0 || i + 1 === arr.length ? splitName[0].toUpperCase() : null
      )
      .join('');
    return (
      <a href={blog} className={classes.link}>
        <Avatar className={classes.avatar}>
          <p className={classes.text}>{initials}</p>
        </Avatar>
      </a>
    );
  }

  return <Avatar className={classes.avatar} />;
};

In the new signup code we're re-using this component to show the GitHub avatar in the signup form, and it's odd to name the URL we pass blog (it isn't a blog).

Let's rename blog to the more generic url, and update all callers to do the same.

Also, blog is an optional prop, but we always use it. We should be returning an <Avatar> without a parent <a> if we don't get passed a url.