Improvements to LGTM download messages

[aeisenberg]

I keep falling into this trap whenever I use the extension to download a database from LGTM.com:

1. I enter the slug/URL.
2. A window pops up in the bottom right with a swirly progress bar.
3. I stare at the progress bar for a bit, wondering "why isn't my download starting?".
4. After a while, I realise that there is an input box waiting for me to pick which language I'm interested in having a database for.

Maybe the "Adding database from LGTM" message should be augmented with "Please pick a language in the dropdown".

cc: @tausbn