Refactor cogs to generalise config access

[Laogeodritt]

Currently, most cogs manually call get_kaztron_config() and store a reference in an attribute, as well as similar calls for the state.json config and any cog-specific config files. For the latter two, many of them also tend to have

Can this be generalised to reduce repetition?

• Base class that loads and provides the config object attribute, as well as direct accessors of some sort for the cog-specific and general (core/discord) sections of the config files?
• on_ready status/precondition on all commands? (pre-command hook - can it cancel command?)
• Abstract base class with deserialise/serialise hooks to load/store data in config?

[Laogeodritt]

Still todo:

• Abstract base class with deserialise/serialise hooks to load/store data in config?
• Universal config elements - output channel, test channel, etc.? ('discord' section of user cfg)
• Abstraction of config section? get_config or similar that abstracts away the section name + __init__ hook for the section name?

Extracted:

• State config - Post-command hooks and some kind of "dirty data" flag to automatically save after a command is processed? (Depends on discord.py 1.0.0) [moved to Auto-save state.json #84]