Fix encoding problems when sharing a question with special characters

Twitter complains with the following error message when sharing a question that contains special characters, such as accented vowels:

```
Invalid Unicode value in one or more parameters
```

The problem is that JS encode() function doesn't handle UTF-8 characters, but encodeURIComponent() actually does.

This PR replaces `escape()` with `encodeURIComponent()` when sharing a question.