expert setting for NOT constraining image width and height

[jrondorf]

Would be nice to have a global setting, which lets you choose if you want to constrain width and height for newly added images or not.

Currently, if you don't want to have it, you have to edit each image after adding it and uncheck these checkboxes.

[yourhead]

moved to: https://gitlab.com/yourhead/stacks/issues/148