webpack commit msg linter, fixes SD-1860

test plan:
* should get a webpack comment on patchset 1

Change-Id: I3fcd5d22b2c9d1b1480958e0ea134e4fefaee714
Reviewed-on: https://gerrit.instructure.com/99354
Tested-by: Jenkins
Reviewed-by: Jon Jensen <jon@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>
QA-Review: Jon Jensen <jon@instructure.com>

script/lint_commit_message

@@ -127,3 +127,13 @@ has_a_test_plan = commit_message =~ /test[ -]plan/i
 unless has_a_test_plan
   comment "warn", 3, "y u no add test plan? ლ(ಠ益ಠლ)"
 end
+
+references_webpack = commit_message =~ /webpack/i
+gerrit_topic = ENV["GERRIT_TOPIC"]
+has_webpack_topic = gerrit_topic && gerrit_topic =~ /webpack/i
+if references_webpack && !has_webpack_topic
+  comment "warn", 3,
+          "looks like your commit is webpack related." \
+          " trigger webpack builds by pushing to the webpack topic:" \
+          " `git push origin HEAD:refs/for/master/webpack`"
+end