Pin md link checker's version (paritytech#7697)

* CI: pin md link checker's version to the previous release

* CI: add a whitelist for allowed GH actions pins

* CI: try master tip [skip ci]

* CI: return to a previous markdown-link-check dependency version [skip ci]

* CI: substitute %20 for a literal space [skip ci]

* CI: substitute %20 for a literal space

.github/allowed-actions.js

@@ -0,0 +1,7 @@
+// This is a whitelist of GitHub Actions that are approved for use in this project.
+// If a new or existing workflow file is updated to use an action or action version
+// not listed here, CI will fail.
+
+module.exports = [
+    'gaurav-nelson/github-action-markdown-link-check@e3c371c731b2f494f856dc5de7f61cea4d519907', // gaurav-nelson/github-action-markdown-link-check@v1.0.8
+]

.github/workflows/md-link-check.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
-    - uses: gaurav-nelson/github-action-markdown-link-check@v1
+    - uses: gaurav-nelson/github-action-markdown-link-check@e3c371c731b2f494f856dc5de7f61cea4d519907
       with:
         use-quiet-mode: 'yes'
         config-file: '.github/workflows/mlc_config.json'

.github/workflows/mlc_config.json

@@ -3,5 +3,11 @@
         {
             "pattern": "^https://crates.io"
         }
+    ],
+    "replacementPatterns": [
+        {
+          "pattern": "%20",
+          "replacement": " "
+        }
     ]
 }