implement backup rotation / limits

[steadfasterX]

According to #337 SeedVault does not have a backup rotation implemented neither by disk space nor a user-chosen one.

Implementation ideas (all of them would be ideal to have, while 2. seems to be a good point to start with):

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]

Related to #566