[bug 1097175] Break out advanced search into a new view.

Like I mentioned in IRC, there is a bunch of duplicate code to deal with after we figure out what we are changing in each view. It might be easier to step through the commits one at a time, since that's how I approached this. I still need to grep through the UI and change all the links to Advanced Search, although they work anyway because we are redirecting.

In the bug I commented:
```
We should test the following:
* Searching from home page (should be across all products)
* Search from product landing pages (should be product specific)
* From a search results page, try the different sidebar filtering options (Help Articles Only, Community Discussion Only, change the product)

That's the important, must not break stuff. In addition, we should test the following less critical pieces:
* From a user profile page, click on the "XX answers" answers link under contributions. Eyeball the search results.
* From the Advanced Search page /search/advanced, do a variety of searches for each of the document types and eyeball the results.
```

r?
Nit: I'd put the class properties before the `__init__` and `clean` methods.