SDP-1445: prevent any html (encoded or not) in the invite templates set by staff users

[marcelosalloum]

What

Prevent any html (encoced or not) in the invite templates set by staff users

Why

[TODO: Why this change is being made. Include any context required to understand the why.]

Known limitations

[TODO or N/A]

Checklist

PR Structure

• This PR has a reasonably narrow scope (if not, break it down into smaller PRs).
• This PR title and description are clear enough for anyone to review it.
• This PR does not mix refactoring changes with feature changes (split into two PRs otherwise).

Thoroughness

• This PR adds tests for the new functionality or fixes.
• This PR contains the link to the Jira ticket it addresses.

Configs and Secrets

• No new CONFIG variables are required -OR- the new required ones were added to the helmchart's values.yaml file.
• No new CONFIG variables are required -OR- the new required ones were added to the deployments (pr-preview, dev, demo, prd).
• No new SECRETS variables are required -OR- the new required ones were mentioned in the helmchart's values.yaml file.
• No new SECRETS variables are required -OR- the new required ones were added to the deployments (pr-preview secrets, dev secrets, demo secrets, prd secrets).

Release

• This is not a breaking change.
• This is ready for production.. If your PR is not ready for production, please consider opening additional complementary PRs using this one as the base. Only merge this into develop or main after it's ready for production!

Deployment

• Does the deployment work after merging?

[stellar-jenkins]

stellar-disbursement-platform-backend-preview is available here:
SDP: https://sdp-backend-pr494.previews.kube001.services.stellar-ops.com/health
AP: https://sdp-ap-pr494.previews.kube001.services.stellar-ops.com/health
Frontend: https://sdp-backend-dashboard-pr494.previews.kube001.services.stellar-ops.com