Add basename and parent_folders fields to Folder graphql interface

[WithoutPants]

Preparation work for a folder filter.

• Adds a basename field to the Folder type. This field is derived from the path field - more on this below.
• Adds parent_folders field to the Folder type, this provides a list of all parent folders for the applicable folder, with the first being the immediate parent and the last being the highest level folder. This means that you can get the full hierarchy of folders (and their ids) with a single query.
• Adds a schema migration to add a basename column. This is necessary for searching folders, since if you search for foo in the path, you will not only get the /foo directory but all of its subdirectories, since foo is part of the path. The basename column is automatically calculated from the path field during creates and updates.
• Adds a basename field to the FolderFilterType input, to allow filtering on basename.