Configurable number of snapshots to keep

Context

This is derived from issue #559 and discussion #964.

From the description of issue #559:

1. auto-rotate (i.e. overwrite oldest backup) when running out of disk space this will likely require to have a rough estimate of the backup size before
2. let the user set a specific backup count most "easy" one as it is up to the user to have an eye on the available disk space
3. let the user set a specific max disk space amount this will likely require to have a rough estimate of the backup size before the backup starts to write

grote said:

For now we'll do 1. with https://github.com/seedvault-app/seedvault/pull/750 By default we keep 3 daily snapshots and 2 weekly ones (one from this and one from last week). For 2. we can open a new ticket.

So this is the ticket for solution number 2.

From the discussion #964, grote said:

Auto-deletion above what it normally deletes isn't implemented and a bit tricky as well. We'd need to warn you anyway, but sometimes continue to back up and sometimes not, depending on how much space we can free. It may be better to allow the user to configure how many snapshots to retain.

The problem

When you're using a very small storage device (because you're poor and unemployed 😭), the default backup rotation mechanism isn't enough to prevent an insufficient-space situation.

Proposal

By allowing a configurable number of backups to keep, we would be able to choose a reasonable number that fits in the device.