Fix default config of account fields length
Problems related to !1488 (merged)
The account_field_name_length and account_field_value_length defaults appear to be too small.
For example, a mastodon account can set to 4 fields and profile text. If URLs are set for these, such as the person's web site, 255 can easily be exceeded, so at least account_field_value_length needs to be larger.
However, account_field_name_length is 512 in this MR, but I think that 255 is sufficient in most cases.
P.S. Thanks to @kPherox for finding this problem.