Reduce getUrl queries [umbrella issue]

A lot of RPC traffic in apps is to get the URLs for attachments and icons. This is unnecessary because the mapping of a blobId or iconId to a URL is deterministic apart from the server port, which currently changes every time the app goes into the background, but once https://github.com/digidem/comapeo-mobile/issues/1364 is addressed, will also be static once the server has started. Due to an unknown issue with RPC on mobile, which is causing slow round-trip RPC requests to the backend, these URL queries are regularly taking >1,000ms, resulting in slow UI rendering.

This is an umbrella issue for removing these requests in stages:

• Keep server port static after startup, implemented via https://github.com/digidem/comapeo-mobile/issues/1364
• Backwards-compatible (e.g. no API changes) generation of URLs in core-react, with a single request to get the server port. See https://github.com/digidem/comapeo-core-react/pull/97
• Separate server from @comapeo/core, and export synchronous URL generation functions or URL templates which can be used in the client, see https://github.com/digidem/comapeo-core/issues/1099
• Return attachments and icons with URLs already included, to remove the need for separate hooks.

Start by reviewing the staged checklist and the linked issues 1364 and 1099, along with core-react PR 97, since no files or tests are named here. Done means the listed URL-query reduction stages are completed without breaking the existing API.