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).

Oh, I would propose to also change the table name to user_x_role (or role_x_user, alphabetically) instead of user_role_linker as that would be more in line with naming conventions, but I figured that would break BC, so left that out.