Add an update script to reset the value

In case encryption was not enabled, we accidently set encrypted = 1 for
files inside mount points, since 8.1.0. This breaks opening the files in
8.1.1 because we fixed the code that checks if a file is encrypted.
In order to fix the file, we need to reset the flag of the file. However,
the flag might be set because the file is in fact encrypted because it was
uploaded at a time where encryption was enabled.

So we can only do this when:
- Current version of ownCloud before the update is 8.1.0 or 8.2.0.(0-2)
- Encryption is disabled
- files_encryption is not known in the app config

If the first two are not the case, we are save. However, if files_encryption
values exist in the config, we might have a false negative here.
Now if there is no file with unencrypted size greater 0, that means there are
no files that are still encrypted with "files_encryption" encryption. So we
can also safely reset the flag here.

If this is not the case, we go with "better save then sorry" and don't change
the flag but write a message to the ownCloud log file.