JetBrains: Add auth feature

Created by: vdavid

We have manual auth right now: the user has to open sourcegraph.com, go to Settings, create an Access token, and add that token to their project sourcegraph.xml.

We want to make this easier for the user.

The possible solutions that I see, in increasing order of effort needed and value delivered:

a.) We leave the process as-is, only document it better. b.) We add a free text field (textbox) to the Settings UI and let the user add an access token. We write docs about the steps. c.) We study Beatrix's solution and add that flow to our plugin. We store the value in sourcegraph.xml and document this to let the user manually change it if needed. d.) We study Beatrix's solution and add that flow to our plugin. We store the setting in a way so that it can also be configured manually on the UI if needed.

As we have time, my vote goes for d.), and the next step I see is to talk to Beatrix and learn the process she created.