Add entry for OSTN15 transformation grid for datum shift

This is the change in plain SQL:

INSERT INTO tbl_datum_transform VALUES (7709, 100018, 4277, 4258, 9615, 'OSTN15_NTv2_OSGBtoETRS.gsb',
  null, null, null, null, null, null, 'UK National Transformation provided by Ordnance Survey',
  'For applications requiring an accuracy of better than 10 centimetres.', 1, 0, 1264);

INSERT INTO tbl_datum_transform VALUES (7710, 100019, 4277, 4326, 9615, 'OSTN15_NTv2_OSGBtoETRS.gsb',
  null, null, null, null, null, null, 'UK National Transformation provided by Ordnance Survey',
  'For applications requiring an accuracy of better than 10 centimetres.', 1, 0, 1264);

(one still needs to manually download the .gsb grid file to use it but at least no need to modify system srs.db)

https://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/formats-for-developers.html