Preview large TXT files fix

Limit the size of the string generating the preview image for TXT files
we also encrypt/decrypt files in the versions folder for previews and if encryption is enabled/disabled
Merge pull request #11392 from owncloud/fix_version_previews

we also encrypt/decrypt files in the versions folder
Merge pull request #11390 from Nemesiz/master

Preview large TXT files fix
Add CssImportFilter. More elegant fix for documents#348
[tx-robot] updated from transifex
Merge pull request #11437 from owncloud/add-import-filter

Add CssImportFilter. More elegant fix for documents#348
adding cssmin and jssmin(minify)
adding argument deleteSelf - if false the directory itself will not be deleted only it's content
adding repair step to clean the asset cache after upgrade + coding style adjustments
fixing return value logic in rmdirr()