v4.7 results in error messages whenever someone joins/leaves a channel, and breaks the compliance export feature

This is breaking the message export feature and is data loss for anyone using/relying on the output of that feature

Customer upgraded their dev server to 4.7 and are seeing the following logs whenever someone joins/leaves a channel:

[2018/02/16 14:50:55 CST] [WARN] Failed to update ChannelMemberHistory table SqlChannelMemberHistoryStore.LogJoinEvent: Failed to record channel member history, pq: column "email" of relation "channelmemberhistory" does not exist
[2018/02/16 14:50:55 CST] [WARN] Failed to update ChannelMemberHistory table SqlChannelMemberHistoryStore.LogJoinEvent: Failed to record channel member history, pq: column "email" of relation "channelmemberhistory" does not exist

Looks like part of Jonathan's fix for bad DB columns got reverted by this PR to fix the release branch after someone accidentally merged master into it https://github.com/mattermost/mattermost-server/commit/c209e4457457edc042f063390c9a222a694f3a6d

The field "Email" got re-added to the struct and table https://github.com/mattermost/mattermost-server/blame/release-4.7/model/channel_member_history.go

But our upgrade code is removing that column at the same time https://github.com/mattermost/mattermost-server/blob/release-4.7/store/sqlstore/upgrade.go#L347