前台网页增加新的 class，修复 footer 显示位置错误的问题

template/oj/account/change_password.html

@@ -1,6 +1,6 @@
 {% extends "oj_base.html" %}
 {% block body %}
-<div class="container">
+<div class="container main">
     <div class="col-md-6 col-md-offset-3">
         <h2 class="text-center">修改密码</h2>
 

template/oj/account/login.html

@@ -1,6 +1,6 @@
 {% extends "oj_base.html" %}
 {% block body %}
-<div class="container">
+<div class="container main">
     <div class="col-md-6 col-md-offset-3">
         <h2 class="text-center">用户登录</h2>
 

template/oj/account/register.html

@@ -1,6 +1,6 @@
 {% extends "oj_base.html" %}
 {% block body %}
-<div class="container">
+<div class="container main">
     <div class="col-md-6 col-md-offset-3">
         <h2 class="text-center">用户注册</h2>
 

template/oj/contest/problems.html

@@ -1,6 +1,6 @@
 {% extends "oj_base.html" %}
 {% block body %}
-<div class="container">
+<div class="container main">
     <ul class="nav nav-tabs nav-tabs-google">
         <li role="presentation" class="active">
             <a href="problem.html">题目</a></li>

template/oj/problem/problem.html

@@ -1,7 +1,7 @@
 {% extends 'oj_base.html' %}
 
 {% block body %}
-<div class="container">
+<div class="container main">
     <ul class="nav nav-tabs nav-tabs-google">
         <li role="presentation" class="active">
             <a href="problem.html">题目</a></li>

template/oj/problem/problem_list.html

@@ -1,6 +1,6 @@
 {% extends "oj_base.html" %}
 {% block body %}
-    <div class="container" ms-controller="problem_list">
+    <div class="container main" ms-controller="problem_list">
         <div class="row">
             <div class="col-lg-9">
                 <div class="row">