Changes:
* start observing weave:service:setup-complete to update the UI after all logins (re-auth, initial sign-in)
* stop observing FxAccountsCommon.ONLOGIN_NOTIFICATION, since the state we depend on to update the UI never changes interestingly as a result of it firing
Changes:
- rename browser-fxaccounts.js "toggle" to onMenuPanelCommand
- rename openSignInPage->openSignInAgainPage and change it to point to about:accounts?signin=true
- introduce openAccountsPage and change the menu panel "signed in"'s UI to call it
- remove period at the end of "Firefox is now syncing" in the success doorhanger
- rename syncErrorPanel.title->syncErrorPanel.heading for consistency
- remove the "Preferences" button from the "you need to sign in again" doorhanger
- "manage"->"change" in the "What would you like to Sync" dialog
- change "Unlink Browser" to "Disconnect", in both the prefs button and confirmation dialog title
- adjust string in the "Disconnect" confirmation dialog
- change "Device"->"computer" in the relink verification dialog
- fix success panel heading styling (should be bold)
