Rendering HTML in alert attributes

[2bithacker]

Issue Summary
In earlier versions of Alerta, we were able to use HTML to mark up text in custom attributes, however in the most recent alerta-webui, these attributes get displayed without rendering.

Environment

• OS: Linux

• API version: 8.7.0

• Deployment: Docker

• For self-hosted, WSGI environment: nginx/gunicorn

• Database: Postgres

• Server config:
  Auth enabled? Yes
  Auth provider? OpenID
  Customer views? No
  (or provide the /config endpoint output)

• web UI version: 8.7.0

• CLI version: 8.5.1

To Reproduce
Steps to reproduce the behavior:

1. send an alert with an attribute containing HTML, such as alerta send -A diff="<pre>diff text</pre>"
2. Go to web UI and look at the alert
3. See that the attribute displays "<pre>diff text</pre>" rather than rendering diff text in a fixed-width font

For web app issues, include any web browser JavaScript console errors.

Expected behavior
The attribute to display diff text in a fixed-width font (due to the <pre> tag.)

This worked in older versions of Alerta, such as 6.5.0.

Additional context
I have a PR to submit to address this issue.