completes #2055

- GET /email/templates returns list of available templates
- GET /email/{caseId}/{templateName} returns template with filled placeholders

Refactor email template endpoints:
- Split existing getEmailTemplates into two dedicated endpoints
- Simplify template list to return only template names
- Improve response structure by including placeholder values separately

- Document template list endpoint with response codes
- Document template detail endpoint with parameters and responses

- added support for forms placeholders
- added mimetype to output, so that a client can determine whether it deals with plain text or HTML
- refactored code to use existing logic / avoid duplication
- issue #2623