Limit the maximum length of a group's description

According to the docs for the Google Directory API for groups (https://developers.google.com/admin-sdk/directory/v1/reference/groups), the maximum length of the description field is 4096 characters. However, according to the docs for the groupssettings API (https://developers.google.com/admin-sdk/groups-settings/v1/reference/groups), that limit is 300 characters. In practice, it appears that the latter is true, so reduce our limit accordingly.