Add initial database migrations for PostgreSQL

migrations/postgres/0001_create_contest.sql

+CREATE TABLE contest (
+       id SERIAL PRIMARY KEY,
+       location TEXT NOT NULL,
+       filename TEXT NOT NULL,
+       name TEXT NOT NULL,
+       duration INTEGER NOT NULL,
+       public BOOL NOT NULL,
+       start_date TIMESTAMP,
+       end_date TIMESTAMP
+)
+

migrations/postgres/0001_create_grade.sql

+CREATE TABLE grade (
+       taskgroup INTEGER,
+       session INTEGER,
+       grade INTEGER,
+       validated BOOL,
+       PRIMARY KEY (taskgroup, session)
+)

migrations/postgres/0001_create_group.sql

+CREATE TABLE usergroup (
+       id SERIAL PRIMARY KEY,
+       name TEXT,
+       groupcode TEXT,
+       tag TEXT,
+       admin INTEGER
+)

migrations/postgres/0001_create_participation.sql

+CREATE TABLE participation (
+       contest INTEGER,
+       session INTEGER,
+       start_date TIMESTAMP,
+       PRIMARY KEY (contest, session)
+)

migrations/postgres/0001_create_session_user.sql

+CREATE TABLE session (
+       id SERIAL PRIMARY KEY,
+       session_token TEXT,
+       csrf_token TEXT,
+       last_login TIMESTAMP,
+       last_activity TIMESTAMP,
+       permanent_login BOOL,
+
+       username TEXT,
+       password TEXT,
+       salt TEXT,
+       logincode TEXT,
+       email TEXT,
+       email_unconfirmed TEXT,
+       email_confirmationcode TEXT,
+
+       firstname TEXT,
+       lastname TEXT,
+       street TEXT,
+       zip TEXT,
+       city TEXT,
+       nation TEXT,
+       grade INTEGER,
+
+       is_teacher BOOL,
+       managed_by INTEGER,
+       oauth_foreign_id TEXT,
+       oauth_provider TEXT
+)

migrations/postgres/0001_create_submission.sql

+CREATE TABLE submission (
+       id SERIAL PRIMARY KEY,
+       session INTEGER NOT NULL,
+       task INTEGER NOT NULL,
+       grade INTEGER NOT NULL,
+       validated BOOL NOT NULL,
+       needs_validation BOOL NOT NULL,
+       nonvalidated_grade INTEGER NOT NULL,
+       subtask_identifier TEXT,
+       value TEXT,
+       date TIMESTAMP
+)

migrations/postgres/0001_create_task.sql

+CREATE TABLE task (
+       id SERIAL PRIMARY KEY,
+       taskgroup INTEGER,
+       location TEXT,
+       stars INTEGER
+)

migrations/postgres/0001_create_taskgroup.sql

+CREATE TABLE taskgroup (
+       id SERIAL PRIMARY KEY,
+       contest INTEGER NOT NULL,
+       name TEXT NOT NULL
+)