Escape double quotes in all strings, not just top level model strings

[kingzacko1]

Description

The original fix for escaping double quotes in Custom HTTP Notifications fixed only top level strings in the model map. However, the backlog is a list of MessageSummary objects so the message.message fields were not properly being escaped. These changes add a JsonSafeEngineProvider which escapes all double quotes in String objects whenever they are substituted into a JSON formatted payload. This should handle all strings in the model, not just those that are top-level values.

Motivation and Context

Graylog2/support#69

How Has This Been Tested?

local tests and unit tests

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[danotorrey]

LGTM and tests successfully