Use PageCounter to keep track of imported pages

What does this MR do and why?

This MR does 2 things:

1. Refactor PageCounter from GithubImport to Import namespace so that it can be reused by other importers.
2. Add PageCounter to Bitbucket Server's UsersImporter.

The PageCounter keeps track of the imported page for a collection (users in this case) for a project so that if the worker fails and retries, we know which page to start importing from. The cache expires if all pages were imported OR after 24 hours.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #433008 (closed)