channel projects (#8456)

Add plumbing for hosted projects. This will currently show them if they
exist
but provides no UX to create/rename/delete them.

Also changed the `ChannelId` type to not auto-cast to u64; this avoids
type
confusion if you have multiple id types.


Release Notes:

- N/A

crates/collab/migrations/20240226163408_hosted_projects.sql

@@ -0,0 +1,11 @@
+-- Add migration script here
+
+CREATE TABLE hosted_projects (
+    id INT PRIMARY KEY GENERATED ALWAYS AS IDENTITY,
+    channel_id INT NOT NULL REFERENCES channels(id),
+    name TEXT NOT NULL,
+    visibility TEXT NOT NULL,
+    deleted_at TIMESTAMP NULL
+);
+CREATE INDEX idx_hosted_projects_on_channel_id ON hosted_projects (channel_id);
+CREATE UNIQUE INDEX uix_hosted_projects_on_channel_id_and_name ON hosted_projects (channel_id, name) WHERE (deleted_at IS NULL);