Reject dangerous changes in Git repositories by default

Summary: Ref T4189. This adds a per-repository "dangerous changes" flag, which defaults to off. This flag must be enabled to do non-appending branch mutation (delete branches / rewrite history).

Test Plan:
With flag on and off, performed various safe and dangerous pushes.

  >>> orbital ~/repos/POEMS $ git push origin :blarp
  remote: +---------------------------------------------------------------+
  remote: |      * * * PUSH REJECTED BY EVIL DRAGON BUREAUCRATS * * *     |
  remote: +---------------------------------------------------------------+
  remote:             \
  remote:              \                    ^    /^
  remote:               \                  / \  // \
  remote:                \   |\___/|      /   \//  .\
  remote:                 \  /V  V  \__  /    //  | \ \           *----*
  remote:                   /     /  \/_/    //   |  \  \          \   |
  remote:                   @___@`    \/_   //    |   \   \         \/\ \
  remote:                  0/0/|       \/_ //     |    \    \         \  \
  remote:              0/0/0/0/|        \///      |     \     \       |  |
  remote:           0/0/0/0/0/_|_ /   (  //       |      \     _\     |  /
  remote:        0/0/0/0/0/0/`/,_ _ _/  ) ; -.    |    _ _\.-~       /   /
  remote:                    ,-}        _      *-.|.-~-.           .~    ~
  remote:   \     \__/        `/\      /                 ~-. _ .-~      /
  remote:    \____(Oo)           *.   }            {                   /
  remote:    (    (--)          .----~-.\        \-`                 .~
  remote:    //__\\  \ DENIED!  ///.----..<        \             _ -~
  remote:   //    \\               ///-._ _ _ _ _ _ _{^ - - - - ~
  remote:
  remote:
  remote: DANGEROUS CHANGE: The change you're attempting to push deletes the branch 'blarp'.
  remote: Dangerous change protection is enabled for this repository.
  remote: Edit the repository configuration before making dangerous changes.
  remote:
  To ssh://dweller@localhost/diffusion/POEMS/
   ! [remote rejected] blarp (pre-receive hook declined)
  error: failed to push some refs to 'ssh://dweller@localhost/diffusion/POEMS/'

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, chad, richardvanvelzen

Maniphest Tasks: T4189

Differential Revision: https://secure.phabricator.com/D7689