feat: add personal projects

personal projects are projects that have no team.

they can only seen by the project members (one of which is whoever first
creates the project).

migrations/0054_remove-team-not-null-constraint-projects.up.sql

@@ -0,0 +1 @@
+ALTER TABLE project ALTER COLUMN team_id DROP NOT NULL;

migrations/0055_add-personal_project_table.up.sql

@@ -0,0 +1,5 @@
+CREATE TABLE personal_project (
+  personal_project_id uuid PRIMARY KEY DEFAULT uuid_generate_v4(),
+  project_id uuid NOT NULL REFERENCES project(project_id) ON DELETE CASCADE,
+  user_id uuid NOT NULL REFERENCES user_account(user_id) ON DELETE CASCADE
+);