Jinja template is not updated when using an alert-specific alert_text

[thib12]

To reproduce, in rule.yml add the following parameters:

alert_text_type: alert_text_jinja
alert_text: "Test"
alert:
  - slack:
      alert_text_type: alert_text_jinja
      alert_text: |
        An abnormal number ({{spike_count}}) of events occurred.

This is done as described in the documentation
You will then notice that your alert on Slack will not use the relevant Jinja template when using the FileRulesLoader.

The most likely cause is that the Jinja template is not recompiled as in https://github.com/jertel/elastalert2/blob/master/elastalert/loaders.py#L469, and therefore a wrong template (the one using the alert_text at the root of the config) is used.

[jertel]

It looks like you could take lines 469-474 and and re-use them after line 522, preferably in a re-usable load_template() function. Feel free to submit a PR if you're interested in seeing this behavior changed for the next release.