issue/980-reader-empty-reblog

Fix #980 and #998 - Reblog activity now prevents reblogging to source blog and displays message when there are no visible blogs that can be reblogged to.

![reblog](https://f.cloud.github.com/assets/3903757/2331746/07e431e2-a44e-11e3-9191-099bb08117e4.png)