WebUI: Sanitize user-input fields of PRs

[itaigilo]

Closes #8203.

Change Description

Adding sanitize for user-input fields of a PR, to prevent malicious attacks. Currently it's only relevant for title + description when creating a PR.

Using the very popular DOMPurify package.

Testing Details

Verified manually that PR creation still works.

[github-actions[bot]]

E2E Test Results - DynamoDB Local - Local Block Adapter

[github-actions[bot]]

E2E Test Results - Quickstart