Bug 966511: Tweak Sync UI, r=ttaubert

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)

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extra : rebase_source : 358c93e1123e11200523b96d97ecd25421edaca4
[Australis] portion of bug 966511: rename gFxAccount.toggle to gFxAccount.onMenuPanelCommand, r=ttaubert

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extra : rebase_source : 999ac3977c3c6f1d397689489b8d9fe8d740c822