[Highlight] Stop caching the wordfilter cog

[Injabie3]

Reloads of the wordfilter cog do not reload it for highlight because it's holding on to the previous reference of the wordfilter cog.

We can either:

1. Always try to fetch the wordfilter cog using the get_cog API.
2. Periodically try to fetch the wordfilter cog (on a timer of some sorts).

1 is preferred.

[quachtridat]

I'm not sure how expensive a get_cog() call can be. If it is expensive and it is called every on_message() event handler call, IMO the 2nd approach is better.