This changes the style of push events that remove tags or branches so
they don't display the commit details. This prevents displaying commit
details such as:

    000000 . --broken encoding

Instead we now simply display the header such as:

    Administrator deleted branch example-branch

This is displayed in the same style as events for newly created
branches/tags.

This commit also ensures that if no commit message is present we simply
don't display anything, instead of "--broken encoding".

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/36685
Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/36722

Remove class from SanitizationFilter whitelist

See merge request !2079

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

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

Refactored HTML parser to avoid duplication of newline parsing.

Closes #2235

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`.