Avoid malicious path traversal in manager's Electron method.
Avoid malicious path traversal in manager's Electron method.

There is an Electron IPC event 'open-image' that accepts any path,
including relative paths that would expose system filenames such as
/etc/passwd.  This change resolves all paths to be rooted at the base
directory for ...server_manager/web_app.

Jigsaw-Code/outline-server,Jigsaw-Code/outline-server,Jigsaw-Code/outline-server,Jigsaw-Code/outline-server