Enhancement: logged in user's profile pic in header

This PR puts the logged in user's profile pic on the far right of the header area (right of the social media icons) if the user is logged in. The pic has the same style as the forum posts: completely circular. If the pic is clicked on, it links through to the user's profile page (but potentially could to go e.g. /my

There is a configuration option to disable this feature.

I added some logic around the social media icons so that if none of them are set on the settings page, nothing is displayed. This prevents the text appearing with no icons underneath it.

I also moved some hard-coded English into the language file.