Add flock to config

This adds a file lock to the config in hope that this prevents race conditions as reported in https://github.com/owncloud/core/issues/11070

Testplan:

- [ ] Delete config.php and make it read-only => Error is thrown that it is not writeable
- [ ] Installation still works
- [ ] Changing config settings works (i.e. using the SMTP config switches in the administration menu)
- [ ] Your PC didn't blow up
- [ ] Installing the news app and the "Disable AppCode checker" app did not destroy your installation

Only skip the main config

Otherwise read only additional configs might not be processed

Test on tmpdir
Use SVG previews for public sharing

Fixes https://github.com/owncloud/core/pull/11367#issuecomment-57306037 and https://github.com/owncloud/core/issues/9218
Redirect only to absolute URL

We do not want to redirect to other domains using the "?redirect_url=" feature. Please notice, that the ownCloud project does not consider open redirectors as security issue.
Fix AmazonS3 rmdir on the root
Fix amazon s3 rename overwrite
Fix copy overwrite for amazon s3
Skip test for unsuported feature for amazon s3
log exceptions when listing files
strict comparison
make some storage tests explain what went wrong
fix amazon s3 issues

folder size and mtime is always unknown in s3

more s3 fixes

make rescanDelay of root dir configurable, add on the fly update of legacy storage ids, !isset -> empty when checking strings

reduce number of http calls on remove and rmdir

fix typo
files_extern: remove empty Body and ContentLength in Amazon S3 mount

fixes #10501

Conflicts:
	apps/files_external/lib/amazons3.php
extract batchDelete(), better comments
fix position and style of sort indicator
Add CssImportFilter. More elegant fix for documents#348
reduce bottom padding in filelist, fix perceived glitching, fix #11213
fix position of rename field, fix #10867
Merge pull request #11457 from owncloud/stable7-fix-rename-position

fix position of rename field, fix #10867
fix for issue #10483
variable for found avatar div