[Feature/RFC] Gate Scene Create button behind config

[ikmckenz]

Currently, many stash-boxes do not allow scenes to be created on their web-UI's, but rather only via API. These are policy decisions, it seems all stash-boxes still have a "Create Scene" button on their UI's, but edits created via that mechanism are rejected. It seems this is done partly because the scene creation UI doesn't allow all scene datapoints, such as hashes (see #751), but also because of the uniformity that relying on the stash code provides.

Admins that do not want scenes created on their web UI's should be able to disable the scene creation button via config.