Arcyd: add user action log to dashboard

It's currently difficult to determine what actions were recently
performed by users of the system.  Explicitly log all actions and
display recent ones on the Arcyd dashboard, this will help give
confidence that the system is working as expected.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh
$ ./testbed/arcyd/loop_test.sh
$ ./testbed/arcyd/status_html_testbed.sh
$ ./testbed/arcyd/test_shell.sh
Arcyd: log git and conduit io to a file

Currently it's hard to piece together what Arcyd has written across all
repos and Phabricator instances, make overview easier by collecting a
summary of all io write operations in one file.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh
$ ./testbed/arcyd/test_shell.sh