* 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 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
