fix migrations for tables with arrays of BigInt

[dantownsend]

Fixes #500

If you had a table containing an array of BigInt, then migrations could fail:

from piccolo.table import Table
from piccolo.columns.column_types import Array, BigInt

class MyTable(Table):
    my_column = Array(base_column=BigInt())

It's because the BigInt base column needs access to the parent table to know if it's targeting Postgres or SQLite.

[codecov-commenter]

Codecov Report

Merging #501 (867cd30) into master (32bd3b1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #501   +/-   ##
=======================================
  Coverage   90.84%   90.84%           
=======================================
  Files         104      104           
  Lines        6998     7000    +2     
=======================================
+ Hits         6357     6359    +2     
  Misses        641      641           

Impacted Files	Coverage Δ
piccolo/table.py	95.66% <100.00%> (+0.02%)	⬆️

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