Updated image branding (issue #4444)

Have created new logos, in lower case Roboto, and changed one file (an emailer) which referred to logo_caps.png. This addresses #4444.

Most of these appear unused in the code. The only two I could find being used were logo_caps.png (now logo_small.png) and powered_by_diaspora.png, which appears in the footer of the desktop version.

I'm wondering whether powered_by_diaspora in the footer wouldn't be better as text rather than an image. The image is so small that it's not easy to read. If we presented it as text in light grey, it would probably look better and also scale better.

But do we even need 'powered by diaspora*' there?