Allow upload ID to contain slashes

[Acconut]

This PR

• adds explicit support for forward slashes in upload IDs (see Allow using '/' when setting ChangeFileInfo.ID to enable "hierarchival" views in Azure Blob Storage #1021 and https://community.transloadit.com/t/tusd-azure-changefileinfo-id-with-folder-support/16871/1),
• changes filestore to create the necessary directories if needed,
• makes the handler capable of processing paths with leading and trailing slashes (see S3 backend with TUS handler fails to handle PATCH #1067 (comment)).