Draft: fix: double protect slashes in jinja2

What does this MR do?

ERROR! We were unable to read either as JSON nor YAML, these are the errors we got from each:
JSON: Expecting value: line 1 column 1 (char 0)

Syntax Error while loading YAML.
  found unknown escape character '/'

The error appears to be in '/Users/andrewn/code/gitlab/dedicated/instrumentor/get/ansible/roles/common_vars/defaults/main.yml': line 44, column 59, but may
be elsewhere in the file depending on the exact syntax problem.

The offending line appears to be:

external_url_sanitised: "{{ external_url | regex_replace('\\/$', '') }}"
external_host: "{{ external_url | regex_replace('^https?:\/\/') }}"
                                                          ^ here
We could be wrong, but this one looks like it might be an issue with
missing quotes. Always quote template expression brackets when they
start a value. For instance:

    with_items:
      - {{ foo }}

Should be written as:

    with_items:
      - "{{ foo }}"

Related issues

Author's checklist

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