Story: API and UIs for creating free write event

[robertu7]

Description

A admin-only way (web-next or OSS) to create a new free write event. That includes:

• APIs of CRUD free write events
• UI of CRUD free write events
• DB updates
• APIS of related noitces

Epic

• https://github.com/thematters/developer-resource/issues/460

Implementations

• https://github.com/thematters/developer-resource/issues/464
• https://github.com/thematters/matters-server/issues/4011
• https://github.com/thematters/matters-server/issues/4012
• https://github.com/thematters/matters-server/issues/4013
• https://github.com/thematters/matters-server/issues/4014
• https://github.com/thematters/matters-server/issues/4015
• https://github.com/thematters/matters-server/issues/4016