Change: Fix three badly styled warning messages

There were 3 labels for warning messages that were nearly unreadable in
dark mode due to bad styling. These have been changed to only use a bold
font, but are now displayed with a big warning icon on their left. The
places are: the account settings page, the folder wizard remote path
page, and the log browser.

https://github.com/owncloud/client/issues/11412
https://github.com/owncloud/client/pull/11470
