* MM-12901 Create showSearchModal action and use for hashtags
* MM-12901 Improve handling of hashtags on Recent Mentions, Flagged Posts, and Permalink screens
* 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.
* Fix reply count in post header
* Make channel loader to trigger manually
* post list selector for search
* Include date separators in search results
* Flagged posts
* Recent Mentions
* Retry option for flagged posts and recent mentions
* feedback review
* Update mattermost-redux
