fix: check EDITOR_MAPPING_PREFIX before appending content type

[ahanniga]

If a custom Streams template is specified in settings, EDITOR_MAPPING_PREFIX can be undefined and throw a Javascript exception. Check if EDITOR_MAPPING_PREFIX is defined. If not, leave editorType unassigned and try best-guess based on encoding.

[saqimtiaz]

@ahanniga Apologies for the late reply. The upcoming TW v5.2.4 release will introduce the $genesis widget which will allow the replacement of the $streams-edit widget with wikitext handling to choose the appropriate editor. The plan is to take advantage of that opportunity to resolve this issue in a more flexible and hackable manner.