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/sqlite/initial_tables.sql

@@ -0,0 +1,27 @@
+PRAGMA auto_vacuum = INCREMENTAL;
+
+-- DROP TABLE IF EXISTS users;
+
+CREATE TABLE IF NOT EXISTS users (
+  id text PRIMARY KEY,
+  username text NOT NULL DEFAULT '',
+  email text NOT NULL DEFAULT '',
+  password TEXT NOT NULL,
+  salt text NOT NULL,
+  confirmation_hash text,
+  created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+  updated_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+  CONSTRAINT chk_username_not_empty CHECK (username != ''),
+  CONSTRAINT chk_email_not_empty CHECK (email != ''),
+  UNIQUE (email),
+  UNIQUE (username)
+);
+
+CREATE TRIGGER IF NOT EXISTS tg_users_updated_at
+  AFTER UPDATE ON users
+  FOR EACH ROW
+BEGIN
+  UPDATE users SET updated_at = CURRENT_TIMESTAMP
+WHERE
+  id = old.id;
+END;