Store statistics on jira imported issues counts

What does this MR do?

This MR stores stats related to a Jira import. Namely it stores count of issues that were imported successfully and count of issues that failed to be imported as well as the total count.

Currently the total count of issues is computed as successfully imported issues + known failed issues. In a future iteration the total count will be fetched from Jira itself, which should account for more accurate report.

This is related both to reporting back to user on the import statistics, as well as historical data on the imports and also can be used as usage ping data for our data analisys.

• re #208920 (closed)
• re #211653

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team