INFRA-6649 Add setup-arc.sh (#34)

This setups up phabricator

Issue: https://khanacademy.atlassian.net/browse/INFRA-6649

Test plan:

Remove ~/.arcrc, run setup-arc.sh and verify everything
still works.

bin/setup-arc.sh

@@ -0,0 +1,41 @@
+#!/bin/bash
+
+# Bail on any errors
+set -e
+
+# Install in $HOME by default, but can set an alternate destination via $1.
+ROOT=${1-$HOME}
+mkdir -p "$ROOT"
+
+# the directory all repositories will be cloned to
+REPOS_DIR="$ROOT/khan"
+
+# derived path location constants
+DEVTOOLS_DIR="$REPOS_DIR/devtools"
+
+# Load shared setup functions.
+. "$DEVTOOLS_DIR"/khan-dotfiles/shared-functions.sh
+
+trap exit_warning EXIT   # from shared-functions.sh
+
+# Set up .arcrc: we can't update this through the standard process
+# because it has secrets embedded in it, but our arcanist fork will
+# do the updates for us.
+setup_arc() {
+    if [ ! -f "$HOME/.arcrc" ]; then
+        echo "Time to set up arc to talk to Phabricator!"
+        echo "First, go make sure you're logged in and your"
+        echo "account is set up (use Google OAuth to create"
+        echo "an account, if you haven't).  Click here to start:"
+        echo "  -->  https://phabricator.khanacademy.org  <--"
+        echo -n "Press enter when you're logged in: "
+        read
+        # This is added to PATh by dotfiles, but those may not be sourced yet.
+        PATH="$DEVTOOLS_DIR/arcanist/khan-bin:$PATH"
+        arc install-certificate -- https://phabricator.khanacademy.org
+    fi
+}
+
+setup_arc           # pre-req: clone_repos
+
+trap - EXIT