New data directory handling

- Use $HOME/Zotero if zotero.sqlite exists, and set it as a custom data
  directory so that 4.0 uses it if loaded (resulting in a
  newer-db-version error instead of an empty database)
- Don't prompt whether to use data directory from the other version's
  profile directory (i.e., Firefox or Standalone) -- just do it

resource/config.js

@@ -1,5 +1,6 @@
 var ZOTERO_CONFIG = {
 	GUID: 'zotero@chnm.gmu.edu',
+	ID: 'zotero', // used for db filename, etc.
 	CLIENT_NAME: 'Zotero',
 	DOMAIN_NAME: 'zotero.org',
 	REPOSITORY_URL: 'https://repo.zotero.org/repo/',