Add account management web pages (#694)

Added internal/frontend/account to implement account management code. 
Added user registration templates and handlers.
Set up a sqlite and postgres database in package main.

internal/data/migrations/postgres/initial_tables.sql

@@ -0,0 +1,16 @@
+DROP TABLE IF EXISTS users;
+
+CREATE TABLE IF NOT EXISTS users (
+  id text PRIMARY KEY NOT NULL,
+  username text NOT NULL DEFAULT '',
+  email text NOT NULL DEFAULT '',
+  password bytea NOT NULL,
+  salt bytea NOT NULL,
+  confirmation_hash text,
+  created_at timestamptz DEFAULT (now() at time zone 'utc'),
+  updated_at timestamptz DEFAULT (now() at time zone 'utc'),
+  CONSTRAINT chk_username_not_empty CHECK (username != ''),
+  CONSTRAINT chk_email_not_empty CHECK (email != ''),
+  UNIQUE (email),
+  UNIQUE (username)
+);