Replace `/ack` API with proposals

Nobody has claimed this yet.

We believe the existing `/ack` paths can be removed, and instead done in the proposal flow. Roughly:

- Remove any consideration of member activeness from the C++ code
- Add a new `ack` action, which requires no votes and always passes resolve, and updates the member's state to `Active`
- When tallying votes in the default `resolve()` function, only count members if they are `Active`

We could also make submit _any_ proposal an implicit ack, so that the proposer doesn't need to submit an `ack` action directly.
This reduces the API surface, and allows services to opt out of the ACK behaviour entirely if they wish, by removing it from their constitutions.

Start by locating the existing `/ack` paths and the C++ proposal flow, especially the default `resolve()` vote tallying. The issue's intended completion is to replace the API with an `ack` proposal action or implicit acknowledgement, remove member-activeness handling from other C++ code, and count only `Active` members when resolving.

Written by the indexing model from the issue text.