pygitweb: simplify per-branch permissions model to not require hooks (only works with smart-http for now)
* remove per-branch pull checks; per-project pull checks remain, and we now have the restriction that users allowed visibility over a project have visibility over all objects in its repo.
* simplify per-branch push checks and move to smart-http instead of hooks.
* this change will make the eventual change from a hook-based workflow to action-based workflow more simple