Add accounts, users, groups tables in database

As well as tables binding these together.

See #16.

storage/migrations/000002_accounts.down.sql

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+drop table accounts;

storage/migrations/000002_accounts.up.sql

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+-- accounts have names and will be used primarily for billing.
+create table accounts (
+  id uuid primary key default gen_random_uuid(),
+  name string not null,
+  created_at timestamp not null default current_timestamp(),
+  updated_at timestamp not null default current_timestamp()
+);

storage/migrations/000003_users.down.sql

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+drop table account_membership;
+drop table users;

storage/migrations/000003_users.up.sql

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+-- users has user information and preferences.
+create table users (
+  id uuid primary key default gen_random_uuid(),
+  name string not null,
+  email string unique not null,
+  password string not null,
+  language string not null default 'en-us',
+  timezone string not null default 'CET',
+  preferences jsonb not null default '{}',
+  created_at timestamp not null default current_timestamp(),
+  updated_at timestamp not null default current_timestamp()
+);
+
+-- account_membership defines who is part of an account.
+-- Note that it's a many-to-many relationship, so one user can be a part of multiple accounts,
+-- and an account can (obviously) have many users.
+create table account_membership (
+  account_id uuid references accounts(id) on delete cascade,
+  user_id uuid references users(id) on delete cascade,
+  primary key (account_id, user_id)
+);

storage/migrations/000004_groups.down.sql

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+drop table group_membership;
+drop table group_ownership;
+drop table groups;

storage/migrations/000004_groups.up.sql

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+-- groups of users are named and have a secret that's used to join the group.
+create table groups (
+  id uuid primary key default gen_random_uuid(),
+  name string not null,
+  invitation_secret bytes not null default uuid_v4(),
+  created_at timestamp not null default current_timestamp(),
+  updated_at timestamp not null default current_timestamp()
+);
+
+-- group_membership defines who's part of a group.
+create table group_membership (
+  group_id uuid references groups(id) on delete cascade,
+  user_id uuid references users(id) on delete cascade,
+  primary key (group_id, user_id)
+);
+
+-- group_ownership defines who owns a group.
+-- Note that a user cannot be deleted if the user is a group owner.
+create table group_ownership (
+   group_id uuid references groups(id) on delete cascade,
+   user_id uuid references users(id),
+   primary key (group_id, user_id)
+);