Bug 966342: Tools menu shows "Sync now" to unverified users in some cases, r=markh

Changes include:
- have gSyncUI._needsSetup check Weave.Service.identity._signedInUser (if already set) and return true if the user is not verified
- make the menuitem hiding/showing code in gSyncUI.updateUI a bit easier to follow

--HG--
extra : rebase_source : b5a99354e13db798ea3d0380391db1993bf36408