* Add paragraph about browser add-ons when encountering some errors

When a crash is caused by a NotFoundError exception, add a paragraph
to the error page mentioning browser add-ons.

Indeed, crashes with NotFoundError are often caused by browser extensions
messing with the DOM in ways React.JS can't recover from (e.g. issues #13325
and #14731).

* Reword error messages

* Add source-mapped stack trace to copyable text in error boundary

* Add the error message to the copied report, not only the stack trace

* 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

* Do not crash the whole UI when loading an invalid column

* Add error boundary component to catch Web UI crashes

* Add stack trace on supported browsers

* Add component stack info, pre-format everything for github

* Make “Reload” a clickable link that calls window.location.reload()

* Remove elephant friend from error boundary, make title stand out more

* Simplify error boundary to only a graphic