Refactor contexts, instead map spacing methods directly to notes

[zachmueller]

Need to stay mindful of any users that may have relied on contexts in its original form and ensure any code changes here properly deduce old style mapping of spacing methods to the new style. This means first trying to parse out the frontmatter properties in the new style, falling back to old settings and mappings between Contexts and SpacingMethods, if they exist, then further falling back to the "default" SpacingMethod (i.e., the first in the list).

Lots to think through here first before moving forward with any changes. How should I delineate between contexts vs spacing methods applied to a given note? How to handle if a user renames an existing spacing method, should I have it run an update to the frontmatter for all affected notes? Parse more thoughts out from my notes on this topic.

Minor changes required as part of this: remove the dropdown input from the Contexts section (since those no longer would get mapped to a specific spacing method). Set contexts.spacingMethodName setting to be optional and put that on path to deprecating the setting entirely.