Add soft wrap option to txt file viewing
### Description
Soft wrapping for the *editor* was [recently added](gitlab-ce!6188). 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.)
issue