Eventbrite/Pinterest Blocks: Extract resolveRedirect as shared utility function

[Copons]

As a follow up to #14075 (see also #14075 (comment)), we should extract the resolveRedirect and resolveUrl functions, respectively from the Pinterest and Eventbrite blocks, and move them into a single shared utility function.

[creativecoder]

Note that the Calendly block now uses the resolve-redirect endpoint as well.

[Copons]

In #14814 I've created a new testEmbedUrl that should make using resolve-redirect simpler.

See longer description: #14814 (comment)

#14814 adds it to Calendly. I'll take care of Eventbrite and Pinterest once the function is merged.