Change NSFW explanation on profile settings page

This PR provides new explanatory text for the NSFW setting, and removes the code which presented different text depending on NSFW setting.

I have made the changes to _edit.haml and _edit.mobile.haml

I have tested both desktop and mobile versions on a local server and everything works - settings page can be updated, and posts are hidden from the stream when NSFW has been selected, and appear again after it's been deselected. However, I would be grateful if someone would check the syntax/coding as this is the first time I've made a PR involving anything more than plain text or HTML changes.