Bump to 0.3.1 with the upstream emote regression fix

Quick patch release. The previous commit on this branch is a
straight cherry-pick of Infi's upstream "Fix a regression from
API 15 updates" (ff899ff), which restores BetterTTV emote
deserialisation by switching the Emote and Top100 DTOs from
public fields to public properties so System.Text.Json picks up
the [JsonPropertyName] attributes correctly.

Without this, a fresh Hellion Chat install would happily fetch
emote JSON from BetterTTV but every entry would deserialise to
empty defaults, leaving the cache silently empty.

Manifest version bumps from 0.3.0 to 0.3.1 in csproj, the
HellionChat.yaml manifest plus its changelog, the custom-repo
repo.json (assembly version, testing assembly version and the
three download links), and the README version banner. The 0.3.0
changelog entry stays underneath in chronological order.

Build (Release) verified clean.

ChatTwo/ChatTwo.csproj

@@ -4,7 +4,7 @@
              0.1.0 is our bootstrap release; the underlying Chat 2 base is
              called out in the yaml changelog so users can see what it
              derives from. -->
-        <Version>0.3.0</Version>
+        <Version>0.3.1</Version>
         <ImplicitUsings>enable</ImplicitUsings>
         <!-- HellionChat fork: assembly is renamed so Dalamud uses
              pluginConfigs/HellionChat instead of pluginConfigs/ChatTwo,