Opal Activities API

## Activities Overview In Opal, "activities" encompass both things like someone adding an asset to content and also someone sending a chat message. In the user interface, these are called "Chat & Activity" but the underlying resource is always an `activity` record. This API currently supports _retrieving_ all types of activity (see the `activity_types` endpoint to retrieve a list of supported types). It only supports _creating_ chat activity. ## Checkpoints The `checkpoints` endpoints are how activities get marked as read for users. The idea is that any activity older than the most recent checkpoint has been seen by the user. ## User @mentions User mentions are represented in the plaintext body of activity messages. You can create them (or interpret them) using the following syntax: ``` @[{{user name}}](user:{{user_id}}) ``` The double-bracketed `{{user name}}` should be replaced with the user's full name (not their "username" or email) and `{{user_id}}` should be replaced with the user's _integer_ id value (not a UUID). Do not use quotation marks or spaces (except between the parts of a user's name). For example, `[John Doe](user:1234)`.