Improve Velero Feature Enhancement Request Template

[stephbman]

Description

Enable users to up-vote or down-vote feature requests and open velero backlog items on the github repository.

Use Case
Users should have the ability to click to up-vote a feature or down-vote a feature to indicate whether this is a feature that is seen as more or less valuable.

The quantity of favorable (up-votes) and non-favorable (down-votes) on a feature will provide a quantifiable way for the velero community to see which features are most and least desired.

Experience
The up-vote and down-vote capability should be added to the feature enhancement request template and should provide an interface that allows the user to click to up-vote and/or click to down-vote.

https://github.com/vmware-tanzu/velero/blob/master/.github/ISSUE_TEMPLATE/feature-enhancement-request.md