Message Center v2

## Goals
**Digests**
- Allow messages to be added to daily, weekly, and monthly digests
- API will generate digest message content just-in-time
- Daily digests send at 5pm bot time
- Weekly digests send on Friday at same time as daily digest
- Monthly digests send on the 1st at same time as daily digest
- Allow users to choose whether they get digests by email, or just in the message center

**Customizable messages**
- Allow us to systematically send rich, dynamic content to users such as how many pending weeds are in the garden, what is available for harvest, when an FBOS update is available, etc.
- Allow users to create customized messages using some combo of HTML, Markdown, handlebars, etc.

**Better message viewing and management**
- Messages get archived, not deleted
- Messages can be bulk archived
- Messages can be collapsed and expanded
- Search
- User controls for which messages they receive in-app and via email

**Other**
- Syndicate blog posts to message center #1714
- Improve message center card formatting #1299

## Mockup