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)
The UX is still pretty bad, but this is marginally better, because
the instructions to "go check your email" remain on the screen.
Related to Bug 949699.
