Prepare v2.19.4

Version bump, release notes, Belarusian and Slovak locales, and the rest
of the midnight-drag work (#253).

Also the fixes from reviewing this branch: EXDATE zone prefixes in the
form the provider actually writes, "only this event" on a row that does
not recur, RDATE carried through the recurrence rewrite, an exact test
for the reminder default sentinel, the widget's re-arm backstop, and the
counts a stopped import reports.

app/build.gradle.kts

@@ -28,8 +28,8 @@ android {
         // which builds this version and then creates the matching vX.Y.Z tag +
         // release itself (versionCode is pinned to MAJOR*10000 + MINOR*100 +
         // PATCH from versionName, e.g. 2.7.2 -> 20702). See docs/RELEASING.md.
-        versionCode = 21903
-        versionName = "2.19.3"
+        versionCode = 21904
+        versionName = "2.19.4"
 
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
     }