[changes] Add an command line option to activate a repo

Summary:
More changes for the VM. When my VM starts up for the first time, i add the changes github repo using command line.

Unfortunately there is no way to set a repo status to active through commandline at the moment. Setting status to active is required for works to fetch revisions and to run tests on indivdual commits. Add this option to command line.

Test Plan:
bin/projects add --label=Changes --repo-url=https://github.com/dropbox/changes
bin/repos edit --activate --backend=git https://github.com/dropbox/changes
bin/repos sync --all

Reviewers: cramer, cf, vishal

Reviewed By: vishal

Subscribers: alexallain

Maniphest Tasks: T26532

Differential Revision: https://tails.corp.dropbox.com/D81897