Show email size in table and mail detail

[EdwinHoksberg]

This commits add a new row to the listview and detail page showing the size of the mail, formatted to a readable string.

No tests because #351 is not solved yet.

[sj26]

Showing the size is a nice idea. I'm not sure it's important enough information to show in both the table and the meta-data. Perhaps just in meta-data.

[Bilge]

It should be shown in the table also to make it easier to compare size differences, particularly considering the horizontal space cost is so minimal.