Handle temporary files being present when loading guild data (#345)

This PR fixes catastrophic guild data loading errors that appear when
there are lingering temporary files. In normal operation, temporary
files are deleted as soon as they are copied to the main file. It is
also expected that temporary files are valid JSON files.

However, due to a yesterday's DoS attack, something™️ happened and a
bunch of empty temporary files got written to disk. When Octobot
recovered from the attack, it was unable to load any guild data because
of the temporary files.

This PR addresses this issue by changing the data loading logic:
1) Check if there's a temporary file. If it exists, try loading it.
2) If it is successfully loaded, move the temp file to the main file and
resume operation as normal
3) If it could not be loaded, try loading the main file
4) If it is successfully loaded, delete the temporary file and resume
operation as normal
5) If it is not, throw an error (like before)

This PR was tested on production data and managed to load every guild
without errors.

Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>

TeamOctolings.Octobot/Extensions/GuildScheduledEventExtensions.cs

@@ -10,7 +10,7 @@ public static class GuildScheduledEventExtensions
         out string? location)
     {
         endTime = default;
-        location = default;
+        location = null;
         if (!scheduledEvent.EntityMetadata.AsOptional().IsDefined(out var metadata))
         {
             return new ArgumentNullError(nameof(scheduledEvent.EntityMetadata));