Add "Clone As" to repositories and generate full clone commands in UI

Summary:
Ref T4175.

  - Add a configurable name for the clone-as directory, so you can have "Bits & Pieces" clone as "bits~n~pieces/" or simliar.
  - By default, use "reasonable" heruistics to choose such a name.
  - Generate a copy/pasteable clone commmand with this directory name.

Test Plan: Looked at some repositories.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T4175

Differential Revision: https://secure.phabricator.com/D8097