don't handle text validation errors for interposed questions
This MR makes the text for interposed questions optional.
@hg13345 @dgrh99 We need to discuss whether subject or text should be optional. In general there are three options:
- Subject optional (this is what this MR implements)
- Text optional (this is suggested by some users).
- both optional (but one must be set)
In addition, i'd suggest to use the "not-twitter-wall" for the students interposed question view so that an empty title wouldn't look weird.