Category logging & reporting improvements

This change request has three parts:
1. Allow manual choice of 'Category’ (same list as for offers/requests) when logging a time transfer not linked to a post. I.e. under the 'post' menu, there should be a 'None' option which then allows choice of a Category manually.
2. Show ‘Category’ as well as ‘Comments’ in the ’Transfers’ report. If the transfer is linked to an offer/request, the report should show the Category of the post, if not linked to an offer/request then it should show the Category manually chosen (if any) during the time transfer.
3. When logging a time transfer, in the 'post' drop-down, it shows offers of the other person (the one who provided the service). Please also show requests of the current user (the person who received the work) and allow one of those to be chosen instead. Some timebanks mostly work from requests rather than offers, so this would be very helpful.