Create detailed ZenDesk tickets from Get Help submissions

[allanlasser]

Closes #2158

• Adds more detail to tickets generated from Get Help:
  • new subject line format: [Category] Problem @username
  • links to user and organization on MuckRock Requests
• Bypasses FlaggedTask when submitting GetHelp forms
• Cleans and validates GetHelp submission using a GetHelpForm class
• Refactors ZenDesk logic out from Task and FlaggedTask, into core/utils.py

Testing steps

1. In the preview deployment, update the env vars so that USE_ZENDESK is true and use ZenDesk credentials from production.
2. In the preview deployment, submit a Get Help form on a request.
3. Check in ZenDesk that the ticket was created correctly (will need to collaborate with @adriensalz on this).

[allanlasser]

✅ Test steps are all passing. We're successfully generating tickets with additional context.