Add size informations on purgelogs form

Hello,

As explained on purgelogs's repository, it will be nice to have size informations for each section (General, Software, etc.) with a sum for all sections. Two size informations should appeared :

• currently size of logs (before purge)
• future size of logs (after purge)

Units for size : SI standard (kB, MB, GB, TB).

With this feature, you have an idea about size of your logs (before and after purge). On top of that, you can identify tables with unexpected size.