Went through message-reducer to see on which actions messageStore.threads needed to be updated, and the two were A. updateThreadLastNavigatedActionType, which we already handled and B. the three send*MessageActionTypes.started actions here.

The logic here to set lastNavigatedTo and lastPruned matches that of https://github.com/CommE2E/comm/blob/master/lib/reducers/message-reducer.js#L984

Verified with logging and Redux DevTools that ThreadActivityStore was updated as expected on SEND_TEXT_MESSAGE_STARTED:

Note that the lastPruned value is being updated here because it hasn't yet been set to initial value. In subsequent diffs will handle adding threadID entry to ThreadActivityStore and removing entry from ThreadActivityStore (eg when user leaves thread). Lot of the work to be done here is interdependent, so there doesn't seem to be an obvious way to sequence things. Starting with the simplest things first, will test thoroughly against messageStore.threads before ripping that out.