sync-webdav improvements

[jim-minter]

• manage remote symlinks (a) to files, and (b) to directories by creating symlinks locally
• set mtimes on directories

[jfenal-zz]

• add videos found on access.r.c (such as https://access.redhat.com/site/videos/647843) List available on page https://access.redhat.com/search/browse/videos/#?

[jim-minter]

@jfenal I've split your RFE out into #76; I don't think that access.redhat.com has an accessible webdav back-end.