Create preview PRs using labels

Benefits:

1. Ease of use - straight from the PR page
2. Workflow runs will show on the PR checks
3. PRs will reflect changes after merging with the base branch (opposed
   to changes on the head branch alone)

.github/workflows/preview_on_auspice_us.yaml

@@ -0,0 +1,20 @@
+name: "Preview on auspice.us"
+on:
+  pull_request:
+    types:
+      - opened
+      - synchronize
+      - reopened
+      - labeled
+
+  workflow_dispatch:
+
+jobs:
+  auspice-us:
+    if: |
+      github.event.label.name == 'preview on auspice.us' ||
+      contains( github.event.pull_request.labels.*.name, 'preview on auspice.us')
+    uses: ./.github/workflows/preview_on_downstream_repo.yaml
+    secrets: inherit
+    with:
+      repository: nextstrain/auspice.us

.github/workflows/make_pr_for_downstream_repo.yaml → .github/workflows/preview_on_downstream_repo.yaml

@@ -1,4 +1,4 @@
-name: "Make PR for a repository which depends on Auspice"
+name: "Preview on downstream repo"
 on:
   workflow_call:
     inputs:
@@ -10,7 +10,7 @@ on:
         type: string
         default: .
 jobs:
-  make-pr-on-downstream-repo:
+  run:
     # I don't see this being used for tags, so ensure it's only run on branches
     # to make subsequent logic and wording easier.
     if: github.ref_type == 'branch'

.github/workflows/preview_on_nextstrain_org.yaml

@@ -0,0 +1,21 @@
+name: "Preview on nextstrain.org"
+on:
+  pull_request:
+    types:
+      - opened
+      - synchronize
+      - reopened
+      - labeled
+
+  workflow_dispatch:
+
+jobs:
+  auspice-us:
+    if: |
+      github.event.label.name == 'preview on nextstrain.org' ||
+      contains( github.event.pull_request.labels.*.name, 'preview on nextstrain.org')
+    uses: ./.github/workflows/preview_on_downstream_repo.yaml
+    secrets: inherit
+    with:
+      repository: nextstrain/nextstrain.org
+      directory: auspice-client

DEV_DOCS.md

@@ -100,12 +100,7 @@ A Heroku pipeline for this repository is connected to GitHub under the nextstrai
 
 #### Test on downstream repositories
 
-Additionally, a GitHub Actions workflow has been set up to generate PRs in downstream repositories that reflect the new changes in Auspice. To use it:
-
-1. Go to [the workflow page](https://github.com/nextstrain/auspice/actions/workflows/make_prs_for_other_repos.yaml).
-2. Select **Run workflow**.
-3. Pick the PR branch, select the downstream repositories you wish to test, and **Run workflow**.
-4. Wait for the workflow to finish running. The Auspice PR should show new reference links from the generated PRs.
+Additionally, a GitHub Actions workflow has been set up to generate PRs in downstream repositories that reflect the new changes in Auspice. To use it, add the label [preview on auspice.us](https://github.com/nextstrain/auspice/labels/preview%20on%20auspice.us) and/or [preview on nextstrain.org](https://github.com/nextstrain/auspice/labels/preview%20on%20nextstrain.org).
 
 ## git-lfs