Add state aid threshold amount to SIC codes.

Certain sectors have a lower state aid threshold,
the default is 200,000 euro.
Show correct SIC state aid threshold when updating lending limit
State aid must not exceed SIC threshold at Loan Entry
Utilise the #sic association.
Merge pull request #225 from alphagov/sic-state-aid-thresholds

SIC state aid thresholds
Add Phase 6 state aid letter.

Includes updates to pre-Phase 6 State Aid Letter
to make both letters more consistent.
We'll have these values by the time the state aid letter is viewed.