Add an option to update the current opened Gist on the Online Editor

The Online Editor can create gists but can't update them. This feature gives the possibility to update a currently opened gist.

It's important to attend to 

• It's necessary to check the ownership of the gist, and compare it with the token owner;
• Handle the filename update. Provide a new URL when the filename is updated and check if the gist already doesn't have the chosen filename;