Rework arena sign data store.

This commit constitutes a major rewrite of how arena signs are stored
and loaded. It fixes an issue where an unloaded or missing world would
cause MobArena to throw errors if there were any arena signs recorded in
said world.

The solution is to load signs of a given world when it is available,
rather than loading all signs at once indiscriminately. At startup,
signs for all _currently available_ worlds are loaded. This fixes the
errors. When a world loads and a WorldLoadEvent is fired, signs in that
world are loaded. This ensures that all valid signs in existing worlds
will _eventually_ load. To keep things tidy, a WorldUnloadEvent will
unload all signs for the unloaded world.

Bukkit's own YAML deserialization implementation doesn't re-throw all
deserialization errors, which means we can't actually catch the problem
of missing worlds without doing an awkward "scan" of the deserialized
objects. This prompted a rewrite of the serialization and data storage
into a custom CSV-like format which is both simpler and also provides a
lot more control over the process. Instead of relying on world _names_,
the new format uses world _UUIDs_. While the rest of the plugin won't
necessarily adapt well to a world being renamed, the signs data store
should be resilient enough to handle it.

Most of the actual sign rendering code and almost all of the template
code is intact, but quite a lot of other classes have been rewritten or
replaced. Some of the rewrites weren't strictly necessary, but because
the components were already fairly small, rewrites were much faster and
a lot less awkward than attempting to adapt existing code.

Fixes #645

changelog.md

@@ -11,6 +11,8 @@ These changes will (most likely) be included in the next version.
 
 
 ## [Unreleased]
+### Fixed
+- Arena signs in unloaded or missing worlds no longer break the startup procedure. Sign data is stored in a new format that MobArena will automatically migrate to on a per-world basis during startup.
 
 ## [0.105] - 2020-11-08
 ### Minor breaking changes