Add support for updating template for CLA Group

[mlehotskylf]

Use Case:
Consider a scenario where we have existing CLA group under an existing project. CLA group has a CLA template with specific terms. At some point, the project leaders decide to update these terms in the CLA template. The challenge is that all existing and new contributors in this CLA group will need to re-sign the CLA with the new terms/template. The goal is to manage this with minimal impact on contributors and contributing organizations.

Proposed Solution:
EasyCLA should provide a mechanism to upload a new CLA template with updated terms and offer an option to request contributors to sign the new CLA within a defined grace period. Ideally, EasyCLA would:

• Allow contributors a grace period to sign the new CLA, during which a warning—such as "Sign New CLA"—would appear in GitHub when they attempt to merge a PR, along with a "cut-off" date.
• If a contributor signs the new CLA within the grace period, they can continue contributing without interruption.
• If they do not sign by the "cut-off" date, their previous "signature" would be invalidated, and they would not be able to contribute until they sign the new CLA.

This approach ensures that contributors have a fair amount of time to comply with the updated terms while minimizing disruption to ongoing contributions.

Alternative solution:
TBD