add context menu on editor line to copy/open web-UI url of a file / line

Each piece of code is uniquely addressable via a URI:
```js
`${protocol}${gitea_host}/${repo_slug}/src/commit/${commit_sha}/${tree_path}#L${line_number_a}-${line_number_b}`
```

It would be nice to have a context menu entry to copy or open this URL in a browser.