Move reading_time to posts table from being calculated on-the-fly

[ibalosh]

One of the fields returned from the API is reading_time. This is not a static field stored in the DB, rather a dynamic field calculated on request. We’d like to make reading_time into a static field stored on the posts table, and returned “normally” from the API.

• Added migration, that will add new column to posts table
• Added migration, backfill, that will update the reading_time column for existing posts
• Updated Posts model, so that we calculate and save reading time to the posts table
• Removed on-the-fly calculation of reading_time for property that is returned by API
• Cleanup (test updates, better names etc)

next step (optional):

• return null on API if there is no reading_time (empty string on front end)
• this would change the response format, but just extend it so it should be fine

Notes

Migration

Everything is in one PR, however, release wise, this would be split up into multiple PR's in following order:

1. migration - to add column to the table
2. upgrade post model, so reading_time is stored in db for new posts, or saves
3. migration - for backfill
   3.1 depending on the size of the table:
   3.1.1 small - do it with this sort of backfill
   3.1.2 big - probably a better choice to do the backfill by separately running the script, use API or SQL queries, to have better control over load on the DB
4. remove on the fly calculation from extra-attrs and use real db column
5. along the way update tests

Tests

Tests only run correctly in CI if PR is in Ghost.