[Vue Rewrite] Explore Page styles and behavior

[devlinjunker]

Related to #748

🗒️ Summary

• Copy styles from old explore.scss file to Vue File
• Use flex to make grid more responsive
• Clicking subscribe button opens AddFeed popover with url pre-populated from feeds.en.json request data

✅ Checklist

• Code is properly formatted
• Sign-off message is added to all commits

- [ ] Changelog entry added for all important changes.

📷 Visual

Using FlexGrid CSS

Clicking Subscribe Button

➡️ Up Next

• Update Documentation for Front End
  • Developer
    • Frontend Tips/Organization
    • Front End Unit Tests
  • Sharing
  • Remove Frontend/JS Plugin Documentation ( should we just update this when we add the new addArticleAction Plugin API?)
• Settings in Sidebar
  • Component
  • Backend Requests + Application Behavior Changes
    • Keyboard Navigation
    • Mark on Scroll
    • Reverse Order (Oldest to Newest)
    • Upload/Download Subscriptions/Articles
• use ?selected=<id> url query parameter to store and parse which Feed Item is displayed in FeedItemDisplay (this will be useful for implementing the search bar - and allow navigating directly to the specific feed item)
  • Also need to update search provider php code
  • Update Documentation?
• addArticleAction Plugin API
  • Nextcloud News Code
  • Add to Bookmarks Example Plugin
  • Add New Frontend/JS Plugin Documentation