Update the PR description for auto webassets udpates

The script for updating webassets uses the commit message from
webapps as the commit message for the PR to teleport.

This commit message is almost always a merged PR, which has the format:

    do some awesome thing (#123)

Where '#123' is the number of the **webapps** PR that was merged.

The problem with this is, when the teleport PR is created, it interprets
the #123 as the number of a **teleport** PR. And since the Teleport repo
has a lot more issues/PRs than webapps, Github ends up linking to an old
and completely unrelated PR.

Fix this by replacing (#123) with (gravitational/webapps#123), which
Github correctly renders as a link to the webapps PR in question.

build.assets/webapps/update-teleport-webassets.sh

@@ -74,7 +74,9 @@ git submodule update --init --recursive
 # set variables based on context from webapps checkout
 BRANCH=$(git rev-parse --abbrev-ref HEAD)
 COMMIT=$(git rev-parse --short HEAD)
-COMMIT_DESC=$(git log --decorate=off --oneline -1)
+# use the commit message from webapps, qualifying references to webapps PRs to that they
+# link to the correct PR from the teleport repo (#123 becomes gravitational/webapps#123)
+COMMIT_DESC=$(git log --decorate=off --oneline -1 | sed -E 's.(#[0-9]+).gravitational/webapps\1.g')
 COMMIT_URL="https://github.com/gravitational/webapps/commit/${COMMIT}"
 AUTO_BRANCH_NAME="webapps-auto-pr-$(date +%s)"