sort dirs/files top-bottom in columns on tree page
Created by: sqs
Previously, tree entries (files and dirs) were sorted left-right, which was hard to read. Now they are sorted top-bottom, which is more familiar and easier to read.
See feedback at https://twitter.com/srcgraph/status/1078011692871757825.
This also removes the usage of CSS grid layout favor of CSS column layout. There are some edge cases, such as column sparseness on wide screens with a small number of entries. To completely fix this, it is necessary to use JavaScript (which is not worth it right now due to the added complexity).