Add 'Boat travel' to list of expense items

When adding a travel expense, there is air, bus, car, and train travel, but no boat travel.

Boat travel is a valid mode of transport for landlords travelling between mainland Europe and the UK, or between parts of the British Isles, to visit their properties.

With the absence of boat travel or a generic travel option, boat travel has to be incorrectly categorised as one of the other travel options. Therefore, one of the following should be done:

1. Add 'Boat travel' as an option.
2. Add 'Generic travel' or 'General travel expense' as an option.
3. Consolidate all travel options into a single 'Travel', 'Generic travel', or 'General travel expense' option.

The last option might be better as the mode of transport isn't important from an administrative or accounting perspective.