Quotas: Some operations fail with a 255 volume type name

On change I6c30a6be750f6b9ecff7399dbb0aea66cdc097da we increased the
`resource` column of the quota_usages table from 255 to 300, because its
value is constructed from (prefix + volume_type_name), but the length of
`volume_type_name` can be up to 255 characters, so if we add a prefix
such as 'volumes_' or 'gigabytes_' to it we'll exceed the db length
limit of the `resource` column.

There are other 3 quota related tables (quotas, quota_classes,
reservations) that have a `resource` column, and they are all
referencing the same kind of thing, but they still have a maximum size
of 255 characters, so there will be things that we won't be able to do
when using a volume type with a 255 characters name. Some of the
operations we won't be able to do are setting a default quota limit for
it or migrate volumes using that volume type.

Related-Bug: #1798327
Related-Bug: #1608849