Fix 500 on users with no name uniphil/commit--blog

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:

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:

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:

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

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