Rethink "metadata" returned from Github module

[triangle-man]

1. Currently we return all values as 'a option. However, some values are not optional.
2. fte_weeks and fte_months should be a single value of a variant type (either FTE_weeks x or FTE_months x.)
3. Most nullable values are nullable because they have defaults. We should replace them with their defaults. (This is not true of the dates)

[mhauru]

Agree with 1. and 2., implementing now.

Confused by 3. The only nullable ones we allow are

1. FTE-months and FTE-weeks, because only one of them may be present, the other should be null or missing
2. Dates
3. Finance codes

Is there a default finance code to use, or should we just use the empty string list as the default? Or were you referring to something else with default values?

[mhauru]

Done