Decouple site expiry from code and database

[supermacro]

Context

When a admin user goes through the site registration flow, they have to prove ownership of the site within a predefined number of days.

Problem

The "predefined number of days" is currently hard-coded into the codebase.

See here:

server/src/routes/admins/sites/get-single.ts

Line 62 in a8d3826

const expiryDay = d.created_at.getDate() + 7

This means that if business requirements change, I need to submit a PR to update that hard-coded value and then deploy the change.

Desired Outcome

Figure out a way to be able to dynamically update the expiry threshold in any environment.

[supermacro]

For reference:

server/src/resources/sites.ts

Line 11 in 8e2e985

export const buildSite = (site: Site): SiteWithExpiry => {