add string interpolators

[erral]

This PR adds 2 string interpolators, intended to be used in content-rule configuration. The interpolators provide access to the Volto portal url and the Volto absolute url of a content item.

These string interpolators are needed when running Plone in a path like /api to be able to configure content-rule actions correctly. For instance, think of a content-rule that sends an email after a content has been added to some folder. This email message includes the URL of the content object added. If Plone is running in /api the URL will include the /api part.

These interpolators use the existing volto.frontend_domain registry setting to produce the frontend url.

These interpolators are only needed when not running in seamless mode, that's why we created them, because back them our Volto was running on / and Plone in /api

Addresses #44

[mister-roboto]

@erral thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[mister-roboto]

@erral thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[erral]

@jenkins-plone-org please run jobs