Fixed normalizePath() to strip out single dot dirs

Now removing "/./" and trailing "/." from the paths when normalizing.
Files app backend now normalizes paths before rendering templates

Before rendering breadcrumbs or the file list, the paths are now
normalized. This prevents the UI to show "." breadcrumbs in case the
path contains sections with "/./"

Fixes #5848
Added missing test with leading backslash
Merge pull request #5862 from owncloud/files-singledotdirs

Files singledotdirs