Create new iati_publisher_frequency table

[akmiller01]

Create a new table in the database as a part of the IATI refresh/reload process such that it captures a timestamp every time a publisher updates their data. This will later allow us to quantify how frequently publishers are updating.

[wakibi]

Is this supposed to capture publisher, codelist and timestamp? Just in case we need to have these stats for other codelists in the future?

[akmiller01]

If we want to imitate the current statistic, it would likely be publisher, most recent transaction date, and timestamp. And then the category is defined as how often they update their most recent transaction date: https://github.com/IATI/IATI-Dashboard/blob/master/timeliness.py

[stale]

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