add tests for calendar scheduling logic; persist sent pings

This also removes sent ping GC'ing on everything but startup. Lack of GC
only becomes a problem if many events are leaking, and we're only going
to ping O(dozens) of events per week.

Dockerfile

@@ -93,7 +93,6 @@ WORKDIR /home/llvmbb_monitor
 
 COPY --chmod=555 docker/run_buildbot_monitor.sh .
 COPY --from=build \
-  /home/llvmbb_monitor/code/target/release/calendar_check \
   /home/llvmbb_monitor/code/target/release/llvm_buildbot_monitor \
   llvm_buildbot_monitor/
 

calendar_check/src/lib.rs

@@ -16,21 +16,22 @@ fn utc_time_to_api_time(time: &UtcTime) -> String {
     time.to_rfc3339_opts(chrono::SecondsFormat::Secs, use_z)
 }
 
-#[derive(Debug)]
+#[derive(Debug, Default)]
 pub enum CommunityEventType {
+    #[default]
     OfficeHours,
     SyncUp,
 }
 
-#[derive(Debug)]
+#[derive(Debug, Default)]
 pub struct CommunityEventDescriptionData {
     pub event_type: CommunityEventType,
     pub mention_users: Vec<Box<str>>,
     pub mention_channels: Vec<Box<str>>,
     pub ping_duration_before_start_mins: Option<u32>,
 }
 
-#[derive(Debug)]
+#[derive(Debug, Default)]
 pub struct CommunityEvent {
     // Unix millis.
     pub start_time: UtcTime,