* Change how CDN_HOST is passed down to make assets build reproducible

* Change webpacker/webpack configuration to dynamically load publicPath based on meta header

* Fix embedded layout missing the cdn-host meta header

Fixes #13882

Refactor and fix #13283, which only worked in some cases.

This two-line change fixes a crash in the front end that occurred
under the following circumstances:
 *  A server had more than one announcement,
 *  A user was displaying the announcements, and
 *  An announcement was deleted (or unpublished, which amounts to
    the same thing.)

As might be expected, the bug was caused by attempting to access a
notification using an index value outside the bounds of the existing
announcements.  Specifically, in two places.  First,
`_markAnnouncementAsRead` attempts to modify announcements based on
the current index.  This is what caused the front end crash.  Second,
when rendering the `Announcements` component, the code paginates the
announcements and displays the current one.  This did not cause a
crash, but caused the front end to confusingly display a blank
announcement (in situations that would have caused a crash) with no
way for the user to navigate back to previous announcements.

This commit fixes both issues by adding a check to ensure that the
code never attempts to access an announcement with an index greater
than or equal to the number of announcements present.

* Add support for links to public statuses in announcements to be opened in WebUI

* Please CodeClimate

* Change meaning of /api/v1/announcements/:id/dismiss to mark an announcement as read

* Change how unread announcements are counted in UI

* Add unread marker to announcements and mark announcements as unread as they are displayed

* Fixups

Change `all_day` to be a visual client-side cue only

Publish immediately if `scheduled_at` is in the past

Add `published_at` and `updated_at` to announcements JSON

* Move announcements above scroll container; add button to temporarily hide them

* Remove interface for dismissing announcements

* Display number of unread announcements

* Count unread announcements accurately

* Fix size of announcement box not fitting the currently displayed announcement

* Fix announcement box background color to match button color

* Add announcements

Fix #11006

* Add reactions to announcements

* Add admin UI for announcements

* Add unit tests

* Fix issues

- Add `with_dismissed` param to announcements API
- Fix end date not being formatted when time range is given
- Fix announcement delete causing reactions to send streaming updates
- Fix announcements container growing too wide and mascot too small
- Fix `all_day` being settable when no time range is given
- Change text "Update" to "Announcement"

* Fix scheduler unpublishing announcements before they are due

* Fix filter params not being passed to announcements filter