Add markdown editor to the admin page

[nigini]

As discussed in #5 we agree that adding a new post editor (like https://github.com/sparksuite/simplemde-markdown-editor) is a desirable improvement, as long as it:

• does not break the page on browsers that are blocking or do not support JS;
• does not mess up with micropub functionalities.

[nigini]

Initial version is added to a secondary /new_post page in this commit.