Use "destinations" rather than "portals"

[philipjohn]

The plugin currently uses the following terminology:

• source = the site content comes from
• portal = the site content goes to

I can't remember why we used "portal" but it doesn't make much sense anymore. I propose:

• source = the site content comes from
• destination = the site content goes to

The plan is to make this change in v1.3. Function and hook names will change! We can add backwards-compatibility though.