Fix to re-download avatars and headers that failed to download

[noellabo]

If the avatar and header download fails for a temporary reason, it will not be updated until the remote_url changes.

For this reason, the image was not updated when the remote account updated information other than the image or when the "REFRESH PROFILE" button was pressed on the moderation account page.

In this PR, if the download fails, it will be recorded in the "needs_download" flag so that it will be re-downloaded if necessary.

In addition to this, you may want to improve "tootctl accounts refresh" or re-download with a scheduled task.

[ClearlyClaire]

Hm, I wonder if we need additional fields… shouldn't {attachment_name}_file_name be nil if the download failed?

[noellabo]

Sure, it's better not to add fields.

Currently, it keeps old icons and headers if the download fails. If you set it to nil, it will be returned to missing.png. Which is the preferred behavior?

[ClearlyClaire]

ah, good point!
I have a preference on returning missing.png rather than a stale image, but this is debatable.

[ClearlyClaire]

I would personally prefer if it set those values to nil instead of adding fields, but @Gargron might have a different opinion.

[Gargron]

I think missing.png makes more sense than displaying stale image (user may not desire old avatar to be displayed anymore)