Make it easier to set common meta data like "description"

All SEO reports tell you to set the title and description meta data, others to set keywords. We should decide what the most common ones are and allow them to be set like the Title (in a simple, customer friendly field).

Even better, we could put the very common ones in like this, and allow a site properties configuration for which other ones the user wants to define on every page, and then dynamically generate a box for each of these to make it super-easy to enter.

The current method really isn't friendly. I know this because it took me 3 attempts to get the right data into this box, and it leaves you having to check the format every time if you aren't a HTML programmer.

Also, we should allow these to be overwritten by plugin output, just like you can with the title. I will then log a separate idea for setting these against products and groups.