Return a non-html-safe, unescaped String instead of ActiveSupport::SafeBuffer
to preserve safety when the output is misused. Currently there's oly one user,
which does the right thing.
Closes#23311
Also preserve code preview color scheme in events dashboard.
Assign default colors to all code blocks shown as <pre class="code highlight [color_scheme]">
Closes#1139
Update the `#first_line_in_markdown` method so that the first line of
parsed text is displayed more reliably, and the continuation indicators
("...") are displayed in all cases where the note is truncated.
Also add Rspec tests for `EventsHelper#event_note`.
