context-menu: Add context menu in left sidebar.
context-menu: Add context menu in left sidebar.

This adds a context menu in the left sidebar. For now, there is one sub-item called - Disconnect organization by which a user can delete an added org from the app.

Fixes: #599.

zulip/zulip-electron,zulip/zulip-desktop,zulip/zulip-desktop,zulip/zulip-electron,zulip/zulip-electron,zulip/zulip-desktop,zulip/zulip-desktop