Run Slack Sync weekly, Skip timesheet notification if taken leave on both halves

[ananyo141]

Please provide enough information so that others can review your pull request:

This PR adds a new feature to sync slack data weekly, and fix an issue with timesheet notification.

Explain the details for making this change. What existing problem does the pull request solve?

• Feat: Run scheduler job weekly to sync slack users (Weekly Sync for New Employee Updates #68)
• Fix: Don't send Please Fill in your timesheet daily reminder if the employee had taken leaves in both halves separately (first and second halves) Bug: Notification sent to employee who applied for half day leaves separately #69

QA List

• Check Scheduler is run weekly to sync Slack with User Meta DocTypes
• Check Please Fill your Timesheet notification is not sent to the user if they have taken leaves separately for both halves.

Screenshots/GIFs

N/A