Show Dgraph version in Ratel

[sleto-it]

It would be useful if we show, in Ratel, the Dgraph version
We have already this info internally, we just need to show it

This will help us a lot especially when one is connected to different versions of Dgraph, on different browser tabs, to identify the version we are working on

A good position could be in the left page, after the 'help' button

Thanks,

[paulftw]

Added as part of 93f4217