- Attach all HTTP handlers to a new `Handlers{}` struct.
- Remove all `handle*` function prefixes.
- Remove awkward, repetitive `app = c.Get("app").(*App)` from all handlers
and instead, simply access it from `h.app` from `Handlers{}`
Originally proposed in #2292.
This commit fully refactors the core campaign manager logic.
It applies a whole new approach to campaign state and lifecycle management.
- Create a new "pipeline" abstraction on top of campaign
for state management.
- Account for every message processed and end campaigns
based on the actual count.
- Discard in-queue messages in the pipeline of a paused
or cancelled campaign.
- `GET /api/events?type=error` opens a long-lived HTTP server side
event connection that streams error messages.
- async (typically SMTP) errors are now streamed to the frontend and
disaplyed as an error toast on the admin UI.
