Remove or mark unreachable resently opened notebooks

[feanor12]

Problem:
The main page of Pluto shows recently opened notebooks, but does not check if they are accessible.
This can confuse people as they don't realize that their moved notebook with the same name does not open anymore.

It would be great to check if the recent files are still there otherwise

• Show a hint that the file is not found / probably moved: disable the entry
• Remove the entry. This could cause problems if the notebook is stored on external storage locations like( external hard drives / network drives)

[pankgeorg]

Hey @feanor12! thanks for using Pluto!
This looks like a duplicate of #877, so let's do the discussion there!