[Github] Truncate Bug Emails when Necessary (llvm#166081)

Mailgun limits the size of an email payload to 16k. Truncate the issue
body, which should be the largest part around 15k and point the user to
Github to see the rest.

Fixes llvm#165020

Author: boomanaiden154

.github/workflows/llvm-bugs.yml

@@ -39,6 +39,12 @@ jobs:
               repo: context.repo.repo
             })
             .then((issue) => {
+              var maybeTruncatedBody = issue.data.body;
+              if (maybeTruncatedBody.length > 15000) {
+                maybeTruncatedBody = maybeTruncatedBody.substring(0,
+                    15000) +
+                  "<truncated>Please see the issue for the entire body."
+              }
               const payload = {
                 author : issue.data.user.login,
                 issue  : issue.data.number,