feat: get rid of da-admin binding

[kptdobe]

Workers binding is pretty cool because it makes code simple and is easy to use.
But... DurableObject is not fully stable on the CloudFlare side (see the recommended error handling with 3 retries... https://developers.cloudflare.com/durable-objects/best-practices/error-handling/).
Also, each object (da-collab worker, da-admin worker, the DurableObject) has its own lifecycle in memory and we can observe random internal errors when using those workers.

To not rely on those bindings, a simple solution is to make a simple fetch on the da-admin endpoint.

This is also a got first step to a da-admin migration on a different infra.

[github-actions]

LCOV of commit 8bf6547 during Install, lint, and test #228

Summary coverage rate:
  lines......: 100.0% (1736 of 1736 lines)
  functions..: 98.3% (59 of 60 functions)
  branches...: no data found

Files changed coverage rate:
                  |Lines       |Functions  |Branches    
  Filename        |Rate     Num|Rate    Num|Rate     Num
  ======================================================
  src/admin.js    | 2.3%     43| 0.0%     1|    -      0
  src/edge.js     | 2.7%    333| 0.0%     9|    -      0
  src/shareddoc.js| 3.1%    617| 0.0%    19|    -      0

[bosschaert]

The changes look good to me.

I see a number of failing unit tests - so I guess this is still work-in-progress 😄