/api/files: add unselect file command

[eltonlaw]

What are the relevant tickets if any?

#3505

What does this PR do and why is it necessary?

Introduces an endpoint to unselect the selected file. The attached ticket mentions it being necessary "so that file queuing mechanisms, for example, might be written" and "to simply unselect the currently-selected file if you made a mistake"

How was it tested? How can it be tested by the reviewer?

Started the server and then ran the following. Seemed to work as expected

>>> import requests
>>> api_token = ...
>>> my_gcode_file = ...
>>> headers = {"Authorization": "Bearer " + api_token}
>>> base_url = "http://localhost:5000/api/files"

# happy path, select and unselect
>>> print(requests.post(f"{base_url}/local/{my_gcode_file}", json={"command": "select"}, headers=headers).content)
b''
>>> print(requests.post(f"{base_url}/local/{my_gcode_file}", json={"command": "unselect"}, headers=headers).content)
b''

# unselect with no file selected
>>> print(requests.post(f"{base_url}/local/{my_gcode_file}", json={"command": "unselect"}, headers=headers).content)
b'Cannot unselect current file when there is no file selected'

# have selected file and try to unselect invalid filename
>>> print(requests.post(f"{base_url}/local/{my_gcode_file}", json={"command": "select"}, headers=headers).content)
b''
>>> print(requests.post(f"{base_url}/local/foo", json={"command": "unselect"}, headers=headers).content)
b'Only the currently selected file can be unselected'

[foosel]

I could have sworn I HAD an unselect command already 😅 Thank you for fixing this oversight! 👍