Bump to 0.3.0 with the audit, brand and command-rename changelog

Phase 2 of the audit follow-ups, the Hellion Online Media brand
sweep and the rename of the slash commands all land in one release.
The slash command rename is breaking for users who had macros bound
to /chat2, /chat2Viewer or /clearlog2, which is the main reason
this is a 0.3.0 rather than a 0.2.1.

csproj, plugin manifest yaml, custom-repo repo.json (assembly
version, testing assembly version and the three download links)
and the README version banner are all moved over together so the
Dalamud plugin list, the manifest and the install instructions
agree. The README project status checklist is updated to reflect
that Phase 2 is closed; Phase 3 holds the remaining backlog
(MySQL backend, encryption, libnotify, etc).

The yaml and repo.json changelogs gain a 0.3.0 block that walks
through the four substantial groups of changes (slash command
rename, audit hardening, brand sweep, About tab) in plain prose.
The 0.2.0 block stays underneath in chronological order.

Build (Release) verified — ChatTwo/bin/Release/HellionChat/latest.zip
(~17.5 MB) and HellionChat.json regenerate cleanly with no warnings.
The tag itself is created by hand alongside the GitHub release.

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>0.2.0</Version>
+        <Version>0.3.0</Version>
         <ImplicitUsings>enable</ImplicitUsings>
         <!-- HellionChat fork: assembly is renamed so Dalamud uses
              pluginConfigs/HellionChat instead of pluginConfigs/ChatTwo,