* was removed in 3e84f170

* used only for windows support
* windows support was dropped in 8.2.0 - see #17379
* was introduced in fd8cb997
* ref #18935

* Version number increased

version.php now contains the previous ownCloud version from which
upgrades are allowed. Any other upgrades will show a message that the
upgrade/downgrade is not supported.

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.

Storage status is saved in the database. Failed storages are rechecked every
10 minutes, while working storages are rechecked every request.

Using the files_external app will recheck all external storages when the
settings page is viewed, or whenever an external storage is saved.

* causes issues on InnoDB because it exceeds the maximal key length
  of an index which is 767 (3-byte charset * varchar(512) = 1536)
* fixes #17619

* file_target is often used in the sharing code in JOIN statements for retrieval of shares

This reverts commit 6a1a4880.

This reverts commit f6a452ca.