Replace JMTE with Freemarker

## Expected Behavior
Graylog should use a single template engine to provide a consistent experience across all features.

## Current Behavior
Currently, we have a mix of JMTE (notifications, decorators, event fields, audit formatting, archive file names, and probably others) and Freemarker (rule builder and sidecar configurations).

## Possible Solution
Since users will have custom templates, we must provide an upgrade path.
The current plan is:
* Add Freemarker support in all places where JMTE is currently available for the user.
* Deprecate the JMTE option in 6.1.
* Automatically convert JMTE templates to Freemarker where possible.
* Prompt user to change over to Freemarker and highlight where conversion failed.
* Remove JTME support in 7.0.

* Graylog Version: 6.0