Consider changing insert_proposals to work with bleeding

[kvashchuka]

Currently insert_proposals supports only release names in the format year.month.version. However, it could support bleeding as well

[jonathan-eq]

I am not sure if we need this after all. I guess insert_proposals is mainly used for non-RMs to upgrade packages as there is less chance to mess something up rather than upgrading the package directly. Upgrade-proposals makes it possible to batch together updates from different teams as they see fit, and then we roll out all the upgrades together.
As I understand it, only (or mainly) RMs should touch bleeding, and it would be more work to go through upgrade proposals. (you would need someone to accept your PR, instead of just running bleeding directly from your branch containing the bleeding upgrades)