MP gain revisions

1. Dailies give MP at baseline rate, unaffected by checklists.
1. Habits affect MP at 25% of baseline. You can lose MP by indulging in bad Habits!
1. To-Dos continue to give MP at baseline with a bonus multiplier from checklist items completed.

"Baseline" is the greater of 1 or 1/10 of the player's max MP.

Also includes some minor refactoring, moving some reused code for gaining MP from tasks into a `gainMP` function within `score`.

Addresses a couple of the checkpoints in https://github.com/HabitRPG/habitrpg/issues/3029.