* MM-11198: avoid duplicate keys in date lines
Ensure each dateline rendered as part of search, recent mentions, or flagged post list has a unique key, even if the same date itself appears multiple times.
* update post_list.test.js to actually test date boundaries, relying on TZ=utc export in package.json
* unit test makePreparePostIdsForSearchPosts
* fix eslint issues
* try TZ=GMT
* encode date lines using timestamps instead
This requires a few extra changes to the post list proper, but largely
simplifies things anyway.
