Improve textarea paste (#31948)

Improve textarea paste (#31948)

- When pasting a URL over another URL, replace the URL instead of
creating a useless `[url](url)`. This is the 1-line change
[here](https://github.com/go-gitea/gitea/pull/31948/files#diff-be8e94d7e3da33b187381f53d28095107bd0cf29ae9a9e997e4f422f4a54479cR122).
- Always run `initTextareaEvents`, previously it was not run when
`dropzoneEl` was not present like when attachements are disabled on the
server. Refactored the function to gracefully handle absent `dropzoneEl`
and rename the function to a better name.