Add RenderedContent admin ui task to delete the cached content

[daveoconnor]

Add an admin interface button that deletes all rows, in batches, using a celery task, and reset the primary key autoincremement value.

[sdarwin]

it might be, there are no major advantages to "in batches".

Here is one hypothetical method:

1. Delete everything at once. Let's say it takes 10 minutes to complete. During this time, rows are being marked as dead. The website is still live, and it may be re-generating cache entries, creating new cache entries at the same time, which is fine.

2. After 10 minutes, the transaction completes. The rows are officially gone.

3. The table now has 50, 100, 150 rows.

4. Here is the chance to reset the primary key. Wait another minute (just to be careful). Then, with only a few rows in the table, delete again. Quickly down to 0. There would not be a 10 minute delay. It would take a second. During this phase, unlike in the first one, it might be possible to "lock" the table, prevent new entries, even crashing django web requests which are hitting the table, but just for a few seconds. Delete the few remaining rows, and reset to 1. Or, again, maybe this part is not even worth the trouble, and don't worry about it.