BrowserView: Rebased and cleaned up

[manavmehta]

What's this PR do? Builds upon #831 which was built upon #793

You have tested this PR on:

• [ ] Windows
• [ ] Linux/Ubuntu
• [ ] macOS (Not tested yet)

** Tasks **

• [X] Add context-menu support
• [X] Logic to handle-external-link
• [X] Check User Agent against a test server

** Temporarily Reverted PR's and commits:** Commits reverted:

• https://github.com/zulip/zulip-desktop/commit/6b7cce0
• https://github.com/zulip/zulip-desktop/commit/8446deb673c88c2b433f9f34ee09f3c1f048d0cf
• https://github.com/zulip/zulip-desktop/commit/983254c310ea1b7454727b11c45028bf95a98dcb

PR's reverted:

• https://github.com/zulip/zulip-desktop/pull/890

[zulipbot]

Heads up @manavmehta, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.