Prevent time entries with end before start

Partial updates of time entries only validated end against start if both
were part of the payload. Start and end are now also validated against the
persisted value if only one of them is sent (fixes #1189).

The importers now fail if the end of a time entry is before the start and
the database consistency check reports such time entries.

app/Console/Commands/SelfHost/SelfHostDatabaseConsistency.php

@@ -59,6 +59,14 @@ class SelfHostDatabaseConsistency extends Command
             ->get();
         $this->logProblems($problems, 'Time entries have a client but no project', $hadAProblem);
 
+        // End of time entries can not be before the start
+        $problems = DB::table('time_entries')
+            ->select(['id'])
+            ->whereNotNull('end')
+            ->whereColumn('end', '<', 'start')
+            ->get();
+        $this->logProblems($problems, 'Time entries have an end that is before the start', $hadAProblem);
+
         // Every user needs to be a member of at least one organization
         $problems = DB::table('users')
             ->select(['users.id as id'])