Files screen: add pinning option to bottom bar when multiple files selected

The third and fourth panes in the [pinning service integration Files screen mockups](https://www.figma.com/file/4cYoTNbjnnV7I7qeOEhjg8/WebUI-Desktop-Pinning-Hi-Fi-Screens%3A-Files?node-id=0%3A1) depict a workflow for setting pinning on multiple files. However, this was de-scoped from pinning integration MVP due to unexpected complexity (orig https://github.com/ipfs-shipyard/ipfs-webui/issues/1500#issuecomment-717483195):

- when user selects multiple items a bottom drawer is displayed with operations that could be applied to all of them
- Q: should we support bulk pin control when multiple files are selected this way?
- A: we could, but **not in scope for MVP**.
- Rationale: this is additional work which needs more thought. We could either:
- pin each selected item separately,
- or we could follow logic used by _Share_ action and create ephemeral directory that contains selected items and only pin the root CID

This issue exists to isolate and implement that work at a later date.
cc @rafaelramalho19 @lidel

Related request from @tcll to implement, regardless of pinning services: https://github.com/ipfs-shipyard/ipfs-webui/issues/1681