[Minor] Use <sup> tag for help icon in web UI page header

This small commit makes the `(?)` web UI help link into a superscript, which should address feedback that the current design makes it look like an error occurred or like information is missing.

Before:

![image](https://cloud.githubusercontent.com/assets/50748/5370611/a3ed0034-7fd9-11e4-870f-05bd9faad5b9.png)

After:

![image](https://cloud.githubusercontent.com/assets/50748/5370602/6c5ca8d6-7fd9-11e4-8d1a-568d71290aa7.png)

Author: Josh Rosen <joshrosen@databricks.com>

Closes #3659 from JoshRosen/webui-help-sup and squashes the following commits:

bd72899 [Josh Rosen] Use <sup> tag for help icon in web UI page header.