Note/hint for entering index name to connect

[cweiske]

Is your feature request related to a problem? Please describe.

When starting dejavu for the first time and it's not clear what we need to do. When we enter the URL http://myserver:9200 nothing happens. It took several confused minutes to realise that, until an appname is entered, the connect button is disabled. It's obvious once you've done it but one idea might be to leave the button enabled (and put it on the left) and post a warning when it is clicked (or Enter is pressed) about missing connection information.

Alternatively, after the URL is entered/changed you could actually load the list of indices as a dropdown box.

This was already described in #270, which has been closed without the issue being resolved.

Describe the solution you'd like
dejavu should list all indices available when only the URL is entered.

Describe alternatives you've considered
The "connect" button should not be greyed out and a warning/error message should be shown when clicking "connect" without an index.