feat(alerting): Make email alert provider e-mail customizable and templateable

[imrelaszlo]

Please see this pull request: #1119

In addition to what is described there, it is possible to send HTML (multipart) emails. To do this, an environment variable must be set for the template folder (GATUS_EMAIL_TEMPLATE_DIR), and for each notification you can configure which template file to use. If there is no template, then use the custom texts, if there is none, then it works according to the original operation (everything is backwards compatible). If template found, plain text body is auto generated (HTML tags stripped out).

        provider-override:
          text-email-subject-triggered: "Alert triggered"
          text-email-subject-resolved: "Alert resolved"
          file-email-body-triggered: "email-template-alert-triggered.html"
          file-email-body-resolved: "email-template-alert-resolved.html"

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• Updated documentation in README.md, if applicable.