Contexts: allow certain functionality under certain conditions

Certain functionality, such as filtering and night mode, are only active in certain contexts: the user is visiting a certain subreddit, frontpage, or all; a post/comment is from a particular subreddit or user; the time of day falls within a certain range, etc.  This PR is the stub of a utility module for centrally managing these contexts in order to address requests like "filter certain subreddits during spoiler-heavy days of the week" as well as adding requested functionality like "toggle switch for certain subreddits or filters".

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If a context is active, the following must all be true:

1. that context has **at least one** associated toggle enabled.
1. FUTURE: the context is within the scheduled time for **all** associated schedules

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Toggles can be associated with a new menu item in the dropdown. If any toggle is enabled, then that menu item is marked "active/on". 

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I've only implemented "context" on keyword filters so far. Before I carry on with implementing it on other features (such as subreddit filters), I was hoping for feedback on the architecture and names (especially "contexts").