add review-bot to require fellows as reviewers

Created a Github Action that uses the [Review-Bot app](https://github.com/paritytech/review-bot) to require fellows to review pull requests before allowing the PR to be merged.

The user's information is fetched always from the chain after every event.

It looks in the fellows data for a field named GitHub and it extracts the handle from there. This resolves polkadot-fellows#7 (you can find more information about the request there)

This uses [`pull_request_target`](https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target) for the event, not `pull_request`. This is a security measure so that an attacker doesn’t have access to the secrets.

.github/review-bot.yml

@@ -0,0 +1,42 @@
+rules:
+  - name: CI Files
+    condition:
+      include:
+        - ^\.github/.*
+    type: basic
+    minFellowsRank: 2
+  - name: Relay files
+    condition:
+      include:
+        - ^relay\/kusama\/.*
+        - ^relay\/polkadot\/.*
+      exclude: 
+        - ^relay\/.+\.adoc$
+    type: basic
+    minFellowsRank: 4
+  - name: System Parachain Files
+    condition:
+      include:
+        - ^system-parachains\/.*
+    type: basic
+    minFellowsRank: 2
+  - name: Target Files
+    condition:
+      include:
+        - ^target\/.*
+      exclude: 
+        - 'README.md'
+    type: basic
+    minFellowsRank: 4
+  - name: General Files
+    condition:
+      include:
+        - '.*'
+      exclude: 
+        - ^relay\/kusama\/.*
+        - ^relay\/polkadot\/.*
+        - ^\.github/.*
+        - ^system-parachains\/.*
+        - ^target\/.*
+    type: basic
+    minFellowsRank: 1

.github/workflows/review-bot.yml

@@ -0,0 +1,32 @@
+name: Review PR
+on:
+  pull_request_target:
+    types:
+      - opened
+      - reopened
+      - synchronize
+      - review_requested
+      - review_request_removed
+      - ready_for_review
+  pull_request_review:
+
+permissions:
+  contents: read
+  checks: write
+
+jobs:
+  review-approvals:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Generate token
+        id: team_token
+        uses: tibdex/github-app-token@v1
+        with:
+          app_id: ${{ secrets.REVIEW_APP_ID }}
+          private_key: ${{ secrets.REVIEW_APP_KEY }}
+      - name: "Evaluates PR reviews and assigns reviewers"
+        uses: paritytech/review-bot@v1.1.0
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          team-token: ${{ steps.team_token.outputs.token }}
+          checks-token: ${{ steps.team_token.outputs.token }}