Facilitate translation comments/notes

[MichaelChirico]

@daroczig mentioned integration with gettext features which facilitate leaving translators' notes / generally communicating between dev & translator with comments. Currently not supported with the tools translation workflow; we could accommodate this with potools for now.

Maybe use #.-style comments to match what's produced in the po file:

https://www.gnu.org/software/gettext/manual/html_node/PO-Files.html

[hadley]

Maybe prepend the last contiguous block of #. comments that occur before the translated string?