lib/model: Handle deleted-then-ignored files (fixes #3502)
lib/model: Handle deleted-then-ignored files (fixes #3502)

When files that were previously marked as deleted became ignored, we
used to do nothing at all. This changes that behavior to set the Invalid
bit (that we should rename to Ignored). This then becomes an update to
other devices that they should not trust our knowledge about the file in
question.

Read this diff without whitespace...

Tested by
- creating a bunch of files on s1
- letting them sync to s2
- shutting down s2
- deleting the files on s1 and rescanning
- adding the files to .stignore on s1 and rescanning
- starting up s2 and letting it sync
- observing the files are not deleted on s2, and it considers itself up
  to date.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3557

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