User tour step content should always be translatable

From MDL-72783 it seems that if the HTML editor is used for user tour step content (in order to include an image) then it is not possible for the step content text to be a language string and translatable.

We don't want to provide a user tour of a new feature with the step title in the user's language and the step content in English.

Thus, let's figure out how to make user tour step content translatable for steps including an image.