Implement support for getting thumbnails from Nextcloud (and maybe OwnCloud. I haven't tested it).
Nextcloud WebDAV access is done through remote.php/dav/files/ while thumbnails are accessed through index.php/core/preview.png, so by checking if the baseURL contains remote.php/dav/files/ we can know if it's a Nextcloud server. If it is, we can replace that with the preview URL.
Also has a minor fix for the existing yandex thumbnail support where file extensions were checked case-sensitively.
Closes #165
Originally posted by @isvvc in amosavian/FileProvider#166