Add option to link to Web IDE to edit files

This extends the edit_on_gitlab helper and adds the option to link to the Web IDE.

• For the Web IDE, use: edit_on_gitlab(@item, editor: :webide)
• For the default editor, use: edit_on_gitlab(@item, editor: :simple)

If no editor is specified, the simple one is used.

I opted to add the Web IDE at the bottom of the page and leave the link in the feedback section to point to the default editor, but we can change that.

This is how it looks locally (unfortunately we hide this for review apps).