Migrate spec/javascripts/blob/ to Jest

Description

Let's migrate the specs in spec/javascripts/blob/ from Karma to Jest.

See the related epic for how to do this.

• spec/javascripts/blob/pdf/index_spec.js (!26894 (merged))
• spec/javascripts/blob/viewer/index_spec.js (!26790 (merged))
• spec/javascripts/blob/blob_file_dropzone_spec.js (!26776 (merged))
• spec/javascripts/blob/notebook/index_spec.js (!27090 (merged))
• spec/javascripts/blob/3d_viewer/mesh_object_spec.js (!26769 (merged))
• spec/javascripts/blob/sketch/index_spec.js (!26889 (merged))
• spec/javascripts/blob/balsamiq/balsamiq_viewer_spec.js (!26655 (merged))
• spec/javascripts/blob/balsamiq/balsamiq_viewer_integration_spec.js (!26889 (merged))