Default: Commit Interval fallback

When importing content, if the user doesn't specify "Do a commit after each number of items", the addon could fallback to a sensible default number.

The default commit interval could be either certain fixed number such as 500. This value could be placed as placed holder in the form field.

Start by locating the import form field labeled "Do a commit after each number of items" and the code that handles its value during content import. Clarify and implement a fixed fallback such as 500, with the form showing the chosen default as a placeholder; done means imports work when the field is left unspecified.

Written by the indexing model from the issue text.