Minor schema changes

- 'default' is a reserved keyword in mysql, so I changed it to
  is_default. I have not seen an occurrence of this column in the
  actual code, so impact seems minimal.
- Although a parent column is present, it did not have a foreign
  key yet, so added that.
- The character set was set to utf8, given that role names are
  something used internally, and the fact that they're used as
  primary keys, I figured they could better be latin1 (reasoning below)
- Because Mysql reserves the maximum amount of bytes for a varchar
  field, it would reserve 3*255 bytes (3 because of charset, 255
  because of maxlenght). This seems a tad overkill, hence I reduced
  it to a more sensible amount (I think).
Renaming `default` to `is_default` in DDL