stashFile on post.remove

`stashFile` was previously only called on `pagehide` (triggered when switching tabs, refreshing or closing a window). Since `sessionStorage` is being used instead of `localStorage`, changes will NOT persist through closing the window and opening a new window, but WILL persist through page refreshes or reloads. This patch adds a call to `stashFile` when the `post` view is removed, allowing changes to persist through navigation events.

#384 An exit warning is displayed when the user attempts to close or refresh the page, although content will be saved on refresh. Exit warning should not be necessary on `post` view removal, as `stashFile` saves content to `sessionStorage` now.

#386 Implementation completed.