Settings UI: allow to independently toggle Markdown for posts and for comments

[eliorivero]

Fixes #6311

See p6TEKc-HQ-jpopdesignp2, comment-2128 by @tyxla

Changes proposed in this Pull Request:

• allow to toggle Markdown for posts and for comments independent of each other
• whitelist option for Markdown for posts.
• remove code in markdown.php that forcefully enables Markdown for posts every time the option is attempted to be written.
• when both are toggled off, the Markdown module is disabled

🗒 the first commit won't be needed if it's merged in #6406

Testing instructions:

• test the markdown options for posts and comments in Writing and Discussion respectively. Each one should work correctly and independently.

[tyxla]

Works like a charm 👍 Thanks for that!

Just a side note: I think we don't really need to disable the module at all - after all, it doesn't do any damage if all post types are disabled, but the module is enabled, right? But that's your call really.

[eliorivero]

It's better to disable it so Jetpack doesn't attempt to load the file.