Added PDFGenerator available check to download PDF button.

- Added PDFGenerator available check to download PDF button before running show/hide or enable/disable commands on it. This was causing JS error running actions when button is not in toolbars.
- Refactored code that was running JS commands on toolbar buttons to show/hide or enable/disable them.

@dclarizio please review/test. This fix should load timelines successfully when PDF button is hidden in the toolbars. 
I'd prefer passing :enabled/:disabled as a symbol over true/false.

Then the method name can be renamed to t1_button_availability or some better name (naming is hard) and you can call it like this:

```ruby
page << t1_button_availability("#{button[0]}", :disabled, "#{button[1]}")
```

I'm horrible at naming but my thought is that :disabled seems more clear in the calling code than false.

It's simple to change the method to look for :disabled or :enabled instead of true/false, but makes the client code more obvious in my opinion.

Thoughts?