Syncing frontend label studio to backend (#4)

## Summary:
Updating the sync frontend code to work with our repositories. This workflow is run under 2 circumstances:

1. A weekly scheduled sync
2. When a PR is merged to the main branch of the label-studio-frontend repository

Frontend testing is done within the frontend repo itself. This workflow buils the docs and static assets for the site (basically replacing the manual sync we need to do in ml-infra).

Also fixing an issue with the notify slack action.

Issue: https://khanacademy.atlassian.net/browse/DI-529

## Test plan:
- ran scheduled sync in workflow testing branch to verify it sends the dispatch message: https://github.com/Khan/label-studio/actions/runs/6255067305
- ran merge job manually in workflow-testing (had to hardcode commit values because repository_dispatch action can only send to the default branch): b9e663e, c3edf97
- deployed ml-infra to test environment off of this workflow-testing branch, verified Jimmy's firefox fix is in: https://github.com/Khan/ml-infra/actions/runs/6255236042

<img width="745" alt="Screenshot 2023-09-20 at 16 46 52" src="https://github.com/Khan/label-studio/assets/50882112/ff8350cb-08ce-408f-b883-02cdce3b78df">

Author: lizfaubell

Reviewers: dat-boris, jimmykodes, lizfaubell

Required Reviewers:

Approved By: dat-boris, jimmykodes

Checks: ✅ Submodules/deps, ✅ Submodules/deps

Pull Request URL: #4

.github/actions/notify_slack/action.yml

@@ -1,11 +1,15 @@
 name: Notify Slack on workflow failure
 description: notifies infrastructure-data-alerts channel on workflow failure.
 
+inputs:
+  webhook_url:
+    description: "Slack webhook url"
+    required: true
+
 runs:
   using: "composite"
   steps:
     - name: Notify slack on failure
-      if: failure()
       uses: slackapi/[email protected]
       with:
         payload: |
@@ -17,4 +21,4 @@ runs:
           }
       env:
         # Notifies the #infrastructure-data-alerts channel.
-        SLACK_WEBHOOK_URL: ${{ secrets.SLACK_WEBHOOK_URL }}
+        SLACK_WEBHOOK_URL: ${{ inputs.webhook_url }}

.github/workflows/follow-merge-lsf-sync-schedule.yml

@@ -1,7 +1,8 @@
-name: 'Follow Merge: LSF Sync'
+name: 'Scheduled Label Studio Frontend sync'
 
 on:
   schedule:
+    # Every Monday at 0400 UTC (before sync to release branch).
     - cron: '0 4 * * 1'
   workflow_call:
   workflow_dispatch:
@@ -12,10 +13,13 @@ jobs:
     steps:
       - uses: hmarr/[email protected]
 
+      - name: Checkout
+        uses: actions/checkout@v4
+
       - name: Create dispatch event
         uses: actions/github-script@v6
         with:
-          github-token: ${{ secrets.GIT_PAT }}
+          github-token: ${{ secrets.KHAN_ACTIONS_BOT_TOKEN }}
           script: |
             const { repo, owner } = context.repo;
             
@@ -38,11 +42,11 @@ jobs:
               repo,
               event_type: 'upstream_repo_update',
               client_payload: {
-                branch_name: 'fb-scheduled-lsf-sync',
+                branch_name: 'develop',
                 base_branch_name: lsf_repo_data.default_branch,
                 repo_name: lsf_repo_data.full_name,
                 commit_sha: lsf_commit.sha,
-                title: "feat: Scheduled LSF sync",
+                title: "Scheduled Label Studio Frontend sync",
                 html_url: lsf_commit.html_url,
                 actor: lsf_commit.commit.author.name,
                 author_username: lsf_commit.commit.author.name,
@@ -51,3 +55,8 @@ jobs:
               }
             });
             return result
+      - name: Notify Slack on failure
+        if: failure()
+        uses: ./.github/actions/notify_slack
+        with:
+          webhook_url: ${{ secrets.SLACK_WEBHOOK_URL }}