Remove legacy forms and views for layer upload, replace and append operations

[giohappy]

Datasets uploads:

• Legacy upload views have been replaced by the new API
• The legacy upload UI is not exposed anymore
• Append and Replace actions are not exposed anymore and the existing views do not play well with the new importer architectures. They will be implemented again according to the new models
• Legacy code is not maintained and can become the source of vulnerabilities

SLD and XML metadata files:

• the current forms have a list of issues. They are basically broken on master branch, since they use legacy code in the backend that has been dropped meanwhile. They should be adapted to the new APIs, but it isn't probably worth it.

For the reasons above the following parts of code will be removed from core:

dataset_upload (including urls)
	-> dataset_upload_metadata 
		-> NewLayerUploadForm
			-> LayerUploadForm
				-> validate_uploaded_files
	-> dataset_style_upload
		-> NewLayerUploadForm
			-> LayerUploadForm
				-> validate_uploaded_files
		
dataset_append (including dataset_append.html template and urls)
	-> dataset_append_replace_view
		-> LayerUploadForm
			-> validate_uploaded_files

dataset_replace (including dataset_replace.html template and urls)
	-> dataset_append_replace_view
		-> LayerUploadForm
			-> validate_uploaded_files

Tests will be adapted acordingly.