Updated "Save Torrent File" button to "Copy Magnet Link" for magnet links

[feross]

Test plan

#7351 (comment)

• Visit https://codepen.io/ferossity/full/qaezaB/
• Click the "magnet" link for any of the given files.
• Verify the following:
  1. The URL bar shows a magnet link
  2. The torrent viewer loads
  3. Clicking "Copy Magnet Link" causes the magnet link to be copied to the clipboard.
  4. Clicking "Start Download" causes the download to start.
  5. Clicking "Copy Magnet Link" still causes the magnet link to be copied to the clipboard.
  6. Clicking on a video/audio file inside the torrent causes streaming to immediately start.

Original issue description

• Did you search for similar issues before submitting this one? Yes

• Describe the issue you encountered: "Save torrent file" button does not make sense for torrents that are started from magnet links. It's not possible to generate a valid torrent file from only a magnet link because we lack the list of piece hashes.

• Platform (Win7, 8, 10? macOS? Linux distro?): All

• Brave Version (revision SHA): master

• Steps to reproduce:

  1. Start a torrent from a magnet link.
  2. Click "Save Torrent File"

• Actual result:

The outputted torrent file is invalid and can't be opened in third-party torrent apps.

• Expected result:

The button should not exist. Or it should be replaced with a "Copy Magnet Link" button.

• Will the steps above reproduce in a fresh profile? If not what other info can be added? Yes

• Is this an issue in the currently released version? Yes.

• Can this issue be consistently reproduced? Yes.

[feross]

This issue is addressed as part of this PR: #7351