DRY release dates and drop dates

[glacials]

Currently we track drop dates separately from release dates in api/internal.json. However every drop date is defined by a release date, so storing both is redundant.

This just contributed to an inconsistency on the website where the two dates didn't match when they should have.

Two example ideas, but there may be more:

1. Replace the drop date field with a reference to a future set, whose release date should determine this set's drop date. The reference could be absolute like a set code, or relative like a number of sets to look forward.

2. Re-introduce some concept of grouping by exit date but without specifying a specific date, e.g. a JSON array, so that each set only has to look to the first set in the next group to find out its own drop date.

Keep in mind rotations usually happen every 4 sets, but not always.