Allow updating default description templates for projects via API

What does this MR do?

This MR allows default issue description and merge request description to be updated via Project API.

Related to #885 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

  • Label as security and @ mention @gitlab-com/gl-security/appsec
  • The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
  • Security reports checked/validated by a reviewer from the AppSec team

Related to #885 (closed)

Edited by Max Woolf