Get login ticket from controller

When using local custom login page,we should pass login_ticket to remote cas server.

sessions_controller.rb
```ruby
#coding: utf-8

class SessionsController < ApplicationController
  before_action RubyCAS::Filter, except: :new
  before_action :init_cas_params

  def new
    @lt = RubyCAS::Filter.login_ticket
    render layout: false
  end

  def destroy
    RubyCAS::Filter.logout(self)
  end
end
```


sessions/new.html.erb
```html
<form action="https://casserver/login" method="post">
      <input type="hidden" name="lt" value="<%=@lt%>">
      <input type="hidden" name="login_method" value="local">
      <input type="hidden" name="service" value="<%=request.protocol%><%=request.host_with_port%>">
       <input class="box_name" type="text" name="username" />
      <input class="box_name" type="password" name="password" />
     <input type="submit"/>
</form>
```

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