Add soft wrap option to txt file viewing

Description

Soft wrapping for the editor was recently added. It seems that it should be even simpler to have soft wrapping for simply viewing .txt files and similar. This would obviate (render unnecessary) sideways scrolling hassles for reading a text file.

Proposal

Add "soft wrapping" for the file viewer as an option. (Possibly make it the default—but if so, the current behavior should definitely be retained as an option.)

Links / references

gitlab-ce!6188

(I don't have any ready examples of public .txt files that have long lines; most public .txt files have hard wrapping.)