A nonce-based Content-Security-Policy thwarts XSS attacks by allowing
inline JavaScript to execute if the script nonce matches the header
value. Rails 5.2 supports nonce-based Content-Security-Policy headers,
so provide configuration to enable this and make it work.
To support this, we need to change all `:javascript` HAML filters to the
following form:
```
= javascript_tag nonce: true do
:plain
...
```
We use `%script` throughout our HAML to store JSON and other text, but
since this doesn't execute, browsers don't appear to block this content
from being used and require the nonce value to be present.
- Turbolinks caches the `head`, so `gon` updates don't show up unless
the user navigates to page directly (by URL) or performs a refresh.
- The solution is to render `gon` in the body instead.
- Also update the syntax to the new Rails 4 (according to the gon
README) syntax.
Stan Hu