Improves how GitHub identities are mapped to OneDev accounts during import.

Issue import

• Adds a login → OneDev user mapping (LoginMapping). Logins that can't be auto-matched by verified email are pre-populated in the import options for manual mapping.
• Applies the mapping to issue submitters, assignees, comment authors and closed_by, and records a proper "Closed" last-activity for closed issues.

Commit / PR-merge attribution

• Previously a person showed up twice: once as their mapped OneDev account (issues), and once as the raw git identity from their commits (e.g. PR merges), because OneDev attributes commits solely via the verified git email.
• New opt-in option "Map commit authors to OneDev accounts" (default on): scans the repo's commits API to link each GitHub login to its commit email(s) and registers them as verified email addresses on the mapped OneDev user, so commits attribute to the same account.
• Emails already owned by another OneDev account are skipped and reported in the import summary. Skips GitHub's synthetic web-flow / [email protected] identities.

Applies to both the project importer and the standalone issue importer. Import summary now reports linked and conflicting commit emails.