Fix UX issues with bug report dialog
* Make it use BaseDialog / DialogButtons (also gives it has a top-right 'x' &
escape to cancel works)
* Stop misusing the 'danger' CSS class on the buttons. There is nothing dangerous
about submitting logs.
* Continued campaign against 'Click here' links.
Fixes https://github.com/vector-im/riot-web/issues/6622
