Use a default gravatar if one cannot be fetched

[sbp]

This PR makes the gravatar endpoint respond with a default PNG if no gravatar can be fetched remotely. The default PNG is just the same image that gravatar uses for its default profile image.

[Humbedooh]

Do we still want a 1 day cache in such cases?

[sbp]

I was thinking that if a gravatar later becomes available then it would be a good idea to reflect it in the UI as soon as possible, but also I don't understand why gravatar itself sometimes returns the default profile image and sometimes doesn't.

[Humbedooh]

I suppose I should clarify: do we want to wait (up to) 1 day in cases where avatar was not found, or do we want a shorter timeout here?

[sbp]

If it's too short then I suppose it hits the server too much, but maybe an hour would be reasonable.