New route param to allow removing a single image from observation

[nimmolo]

Preparation for revised obs form - this route is as yet unused in the front end.

Allows detaching an image from an obs by sending a simple PUT request to
/observations/:id/images/:image_id/detach
with no other parameters.

Includes new test.

[coveralls]

coverage: 94.429%. remained the same
when pulling 666f749 on nimmo-try-single-image-detach-route
into a45859f on main.

[coveralls]

coverage: 94.429%. remained the same
when pulling 99bf823 on nimmo-try-single-image-detach-route
into a45859f on main.

[coveralls]

coverage: 94.429%. remained the same
when pulling 99bf823 on nimmo-try-single-image-detach-route
into a45859f on main.