Split `issuable_client.js` type policies into general and app-specific ones

Currently, we use issuable_client.js to store type policies for ALL the applications that use it. However, some of the policies should be specific to boards, some to issuable lists and some to work items. We need to move the app-specific policies to app's index.js file and define them via addTypePolicies:

apolloProvider.clients.defaultClient.cache.policies.addTypePolicies({
  EpicConnection: {
    merge(existing, incoming) {
      return incoming;
    },
  },
});