Hide seen transactions by default in all modern applications

Summary:
Ref T2222. This restores the "N older comments are hidden." shield to all ApplicationTransactions applications. Roughly the rule this uses is that transactions older than your most recent comment are hidden, under the assumption that you've already read and dealt with them, since you replied afterward. Then we show your last comment to remind/contextualize you, and anything afterward. We also don't hide transactions if we'd only be hiding a handfull, and we never hide the few most recent transactions.

This might need some #design help.

Test Plan:
The tricky part here is the anchor rule, which deals with the case where you follow a link to `T123#4`, but that would normally be hidden. We simulate a click on "show all" if you hit an anchor which is hidden. Here's what it looks like in Maniphest:

{F112891}

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8229