Allow only certain users to make large commits (megabytes) (BB-9416)

If a user makes a large commit (hundreds of megabytes) it can be hard to fix it, as everyone downstream may be required to manually rebase their own work.

(And if anyone forgets to rebase, they might push the large commit back to the repository again, starting the work all over again.)

We'd like to set a threshold size for a given commit. If the commit is greater than N bytes, it should only be allowed to be pushed by an administrator.