JetBrains: Allow setting a default remote

Created by: vdavid

Problem

If there are multiple remotes for the current repo, the {right click on editor} → Sourcegraph → Copy link to file feature feature doesnn't know which repo to link to.

Solution

@abeatrix wrote about the VS Code extension: "We actually have a setting sourcegraph.defaultBranch where you can set the default branch!" That'd be perfect for the JetBrains plugin as well.