Fix: Prevent stacking of Effects

[paullessing]

Fixes #16

It seems that for some reason, the code managed to get itself into a state where the previously applied Effects were not recognised as having already been applied. As a result, Effects were not being removed when there were duplicates if the Effect tier was the same as the previous tier, which caused massively stacking effects.

This change updates the code so that regardless of the existing tier, we always delete other Effects (there can only be one). Then we update the effect to match what the encumbrance should be.

The core part of this code is in commit 5c4dd44 which can be reviewed standalone; the remainder of the commits are simplifications to the code. I can drop these commits if you don't feel they are appropriate.

[paullessing]

This bugfix does not seem to work; I had the issue reoccurring in a game last week. I suspect it's caused by a race condition when two people (in my case DM and a player) have the same character sheet open. Will sort this weekend.