Installer support for non-512-byte sector sizes

[vjr]

See pop-os/distinst#181

Noticed this while working on #447

[vjr]

Looks like installer code already handles/uses the distinst disk get_sector_size () value, except for the https://github.com/elementary/installer/blob/master/src/Objects/Mount.vala class.

This may only be relevant for the EFI ESP partition size check, in the (rare?) occasion the sector size is not 512 bytes, so this issue is probably low priority.

[dr-Styki]

+1 With 4k sector size, a 256GB SSD is shown as a 32GB SSD

Same SSD with 512b sector size

[patx]

This should be Priority: High