Added the ability to edit the meta description tags + fixed some small issues on edit_page

I added the ability to edit the meta description tags. The description is optional, so if the user doesn’t fill the `Description Field` then the description will be an empty string. The description is limited to 160 characters so it won’t be truncated by the search engines. 

I also fixed some small issues on the edit_page. Every time I edited the page, the text inside the text fields didn’t update after I clicked the submit button. And also every time I changed the `File Name`, the page didn’t redirect to the new URL, and as a result if I tried to edit the page once again I got a 404 error.