Use "destinations" rather than "portals"

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.

Search the plugin for uses of “portal,” including function and hook names, and trace how source and destination terminology is exposed. Confirm the full rename scope and decide which old names need backwards compatibility; done means v1.3 consistently uses “destination” without breaking supported integrations.

Written by the indexing model from the issue text.