revert: restore conversation unread count feature flag (#14623)

This reverts #14610 so conversation unread counts are again controlled
by the `conversation_unread_counts` feature flag across the API,
ActionCable broadcasts, notifier/listener paths, and dashboard sidebar
fetching.

## Closes
- None

## What changed
- Restores feature-flag checks for conversation unread count reads and
broadcasts.
- Restores the dashboard feature flag constant and sidebar/store
behavior for disabled unread counts.
- Restores the specs that cover disabled-feature behavior.

## How to test
- In an account with `conversation_unread_counts` enabled, verify
sidebar unread counts are fetched and updated in real time.
- Disable `conversation_unread_counts` for the account and verify unread
count requests/broadcasts are skipped.

app/javascript/dashboard/featureFlags.js

@@ -46,6 +46,7 @@ export const FEATURE_FLAGS = {
   COMPANIES: 'companies',
   ADVANCED_SEARCH: 'advanced_search',
   CONVERSATION_REQUIRED_ATTRIBUTES: 'conversation_required_attributes',
+  CONVERSATION_UNREAD_COUNTS: 'conversation_unread_counts',
 };
 
 export const PREMIUM_FEATURES = [