Tracks some additional Helpshift events.

[diegoreymendez]

Fixes #3470.

Also removes events WPAnalyticsStatSupportSentMessage and WPAnalyticsStatSupportSentReplyToSupportMessage since:

1. There was no way to track those, and
2. They kind of overlap with WPAnalyticsStatSupportReceivedResponseFromSupport which is already tracked.

/cc @sendhil - Ready for review

[diegoreymendez]

Doubt: even if the enum name changed for WPAnalyticsStatSupportSentMessage we may want to keep the original property flag.

Thoughts?

[diegoreymendez]

/cc @astralbodies - Can I ask you for a review?

[diegoreymendez]

@frosty - Can I bother you with a review?

[frosty]

@diegoreymendez Other than the conflict with Podfile.lock, the changes look good to me! Regarding changing the flag, I don't feel I know enough about our analytics setup to really make a recommendation. I like the fact that if we change it, it's consistent with the other new flags. Also, it looks like it's been changed on Android, so perhaps we should follow their lead.

[diegoreymendez]

Ty @frosty !