(From the user feedback interview.)
This would be useful. We don't want to do "All projects", just all wikipedias.
Checking the "All wikipedias" checkbox will disable the input box above it, in JS.
Server-side we will discard if that input box supplies us anything. We probably want to store a value like "all" in `ew_dbname` field or we'll end up adding hundreds of rows for each event. This will probably change a few things in how event queries are done.
Also if someone unchecks "All wikipedias" and adds wikis manually, we should take that row out.
In the event page UI, only show wikis which have any data in pages created/improved. If not, don't display it.