* diagnostics_test.go: fix spacing
* diagnostics_test.go: explicitly assert payload
This fails, since the package is currently receiving struct pointers and won't set the MessageId or Timestamp on the corresponding Message.
* MM-18115: fix segment v3 usage
In v5.14, we updated [github.com/segmentio/analytics-go](https://github.com/segmentio/analytics-go) to v3 as part of https://mattermost.atlassian.net/browse/MM-12389. As noted in the [migration guide](https://segment.com/docs/sources/server/go/#migrating-from-v2), the API subtly changed to expect a struct value and not a struct pointer:
```go
// in v2, you would call the `Track` method with a `Track` struct.
client.Track(&track)
// in v3, you would call the `Enqueue` method with a `Track` struct.
// Note that a pointer is not used here.
client.Enqueue(track)
```
Unfortunately, we kept passing a pointer, and the package didn't complain since it only required an interface -- which the pointer to these structs still implemented. Internally, it only checked for the value types, and failed to annotate our payloads with the requisite metadata. Upstream, segment.io accepted the payload, but then discarded it silently.
This has since been reported and fixed in https://github.com/segmentio/analytics-go/pull/146, but isn't yet part of a tagged release of the package.
Fix our code to pass struct values instead.
Fixes: MM-18115
* Convert app/notification_email_test.go t.Fatal calls into assert/require calls
* changes as per review
* using require.Regexp instead of require.True
* Add different types for different mentions
* Remove redundant THREAD_ANY and THREAD_ROOT constants
* Make PostStore.Get return thread in order
* MM-17071 Add initial version of countMentionsFromPost
* MM-17071 Refactor comment mention counting
* MM-17071 Use mention counting when marking post as unread
* Fix shadowing in tests
* Remove repeated check of user count
* Refactor code using MentionType
* Update comments around -1 return value
* Move inner functions out of countMentionsFromPost
* Remove preconditions check as separate test case
* Update comments
* Add User.GetMentionKeys
* Revert "Make PostStore.Get return thread in order"
This reverts commit 22aa010cee359655fe75e1dc899cf0ffb0943c2a.
* Fix tests
* Fix merge conflict
* Add store.MentionAllPosts
