Fix HTTP POST 2, now json-escaping all strings replaced by the jinja2 template

[ddurham2]

Description

For HTTP POST 2, now json-escaping all strings replaced by the jinja2 template in the json which otherwise leads to a syntax error in the json which will not deserialize properly. See #896 for more details on what was happening.

Will it break anything? Before the fix, if a matched event's value contained a newline (or any other control chars that json escapes) and if that value was included in the jinja2 template, the json of the post body could not be generated, because it would internally attempt to deserialize a now-invalid json string.

If anyone else in the field had encountered this problem, they may have worked around it by including already-escaped values before sending into elastic search (which would affect any other downstream processors of the event, needing to know to undo that hack to the value). This fix should address the internal-processing problem in elastalert once and for all, but users would need to read the changelog and stop doing their work-around as they upgrade.

Checklist

• I have reviewed the contributing guidelines.
• I have included unit tests for my changes or additions.
• I have successfully run make test-docker with my changes.
• I have manually tested all relevant modes of the change in this PR.
• I have updated the documentation.
• I have updated the changelog.

Questions or Comments

No changes needed to docs necessary.

[jertel]

Looks good, thanks again!