Add force push prevention checks for pull requests

[jayam04]

Explanation

Checklist

• I have successfully deployed my own instance of Oppiabot.
  • You can find instructions for doing this here.
• I have manually tested all the changes made in this PR following the manual tests matrix.
• Manual test: Tests force-pushes being closed by Oppiabot. jayam04/oppia#103
  1. Close PR when author force-pushes the commit.
     
  2. Close PR when author re-opens closed PR due to force-push.
     

[jayam04]

Matrix Testing

1. Error logs from the Oppiabot

   [@octokit/app] Deprecated. Use @octokit/app-auth instead. See https://github.com/octokit/app.js/#deprecated
   (node:429126) [DEP0040] DeprecationWarning: The `punycode` module is deprecated. Please use a userland alternative instead.
   (Use `node --trace-deprecation ...` to show where the warning was created)
   New Relic for Node.js was unable to bootstrap itself due to an error:
   Error: New Relic requires that you name this application!
   Set app_name in your newrelic.js or newrelic.cjs file or set environment variable
   NEW_RELIC_APP_NAME. Not starting!
       at createAgent (/home/jayampatel/oppiaroot/oppiabot/node_modules/newrelic/index.js:146:11)
       at initialize (/home/jayampatel/oppiaroot/oppiabot/node_modules/newrelic/index.js:87:15)
       at Object.<anonymous> (/home/jayampatel/oppiaroot/oppiabot/node_modules/newrelic/index.js:37:3)
       at Module._compile (node:internal/modules/cjs/loader:1565:14)
       at Object..js (node:internal/modules/cjs/loader:1708:10)
       at Module.load (node:internal/modules/cjs/loader:1318:32)
       at Function._load (node:internal/modules/cjs/loader:1128:12)
       at TracingChannel.traceSync (node:diagnostics_channel:322:14)
       at wrapModuleLoad (node:internal/modules/cjs/loader:219:24)
       at Module.require (node:internal/modules/cjs/loader:1340:12)
       at require (node:internal/modules/helpers:138:16)
       at Object.<anonymous> (/home/jayampatel/oppiaroot/oppiabot/index.js:19:1)
       at Module._compile (node:internal/modules/cjs/loader:1565:14)
       at Object..js (node:internal/modules/cjs/loader:1708:10)
       at Module.load (node:internal/modules/cjs/loader:1318:32)
       at Function._load (node:internal/modules/cjs/loader:1128:12)
       at TracingChannel.traceSync (node:diagnostics_channel:322:14)
       at wrapModuleLoad (node:internal/modules/cjs/loader:219:24)
       at Module.require (node:internal/modules/cjs/loader:1340:12)
       at require (node:internal/modules/helpers:138:16)
       at exports.resolve (/home/jayampatel/oppiaroot/oppiabot/node_modules/probot/lib/resolver.js:11:12)
       at Probot.load (/home/jayampatel/oppiaroot/oppiabot/node_modules/probot/lib/index.js:218:32)
       at /home/jayampatel/oppiaroot/oppiabot/node_modules/probot/lib/index.js:239:78
       at Array.forEach (<anonymous>)
       at Probot.setup (/home/jayampatel/oppiaroot/oppiabot/node_modules/probot/lib/index.js:239:38)
       at Function.<anonymous> (/home/jayampatel/oppiaroot/oppiabot/node_modules/probot/lib/index.js:188:32)
       at step (/home/jayampatel/oppiaroot/oppiabot/node_modules/probot/lib/index.js:33:23)
       at Object.next (/home/jayampatel/oppiaroot/oppiabot/node_modules/probot/lib/index.js:14:53)
       at fulfilled (/home/jayampatel/oppiaroot/oppiabot/node_modules/probot/lib/index.js:5:58)
   

Testing Steps

1. Created 12 branches with conflicts in 1 new file.
2. Pushed 10 branches with conflicts. (And opened PRs -- 10 with conflicts and 2 older ones).
3. Ran Oppiabot locally.
4. RESULT: The required label was added to all 10 branches.
   
5. Merged the development locally and fixed conflicts. Then, pushed the changes.
6. RESULT: The label was removed.
   
7. Stopped Oppiabot
8. Fixed conflicts in 3 branches and pushed the changes.
9. RESULT: No labels removed.
10. Started Oppiabot
11. RESULT: Oppiabot didn't remove the label.
    
12. While Oppiabot is running, resolved the merge-conflict in one branch and pushed the changes.
13. RESULT: Oppiabot removed the merge-conflict label (check Branch 5).
    
14. Created a new PR with exisiting conflicts when Oppiabot was already running.
15. RESULT: Oppiabot didn't add the merge-conflict label.