Fix 500 on users with no name

Some users have usernames, but not names on GitHub (like @rileyjshaw used to).

On commit--blog, everywhere there should be a name, I do `user.name or user.username`. If name is `None`, it'll fall back on their username, which must exist.

That pattern now pops up in a bunch of places so a better fix would be to add a method to the user object to just get the appropriate one. So this isn't a very good fix but I'm lazy.

This commit should fix #24, however. In that case a string was being concatenated to the username with the `+` operator, something like:

```python
greeting = 'hello ' + user.name or user.username
```

This doen't work at all when `user.name` is `None`, because in python, the `+` operator has a higher precedence than the `or` operator. So it's effectively doing:

```python
greeting = ('hello ' + user.name) or user.username
```

... which is clearly not the desired effect, and moreover string concatenation with `None` is illegal so it breaks.

The quick fix here is to force precedence with parens, like:

```python
greeting = 'hello ' + (user.name or user.username)
```

though maybe string concatenation is a poor patern and I should have done string formatting. Whatever.