Github Importer: Validate repository size before importing

What does this MR do?

Currently the size of the github repository is not validated before starting the importing process. This leads to a slow feedback loop where the failure happens much later in the process. To improve the feedback loop and give the user better experience the validation is now done before the import starts. If the repository to be imported is larger than the namespace or application limits an error message expressing the repository size and the limit size is shown.

Related to: #215057 (closed)

Screenshots (strongly suggested)

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