Preserve link href in truncated note view

Notes on the dashboard views are truncated to 150 characters; this
change ensures that when a link's text is truncated it still points to
the correct URL.
Improve dashboard note view and add tests

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`.
Merge pull request #8020 from mr-vinn/note-trunc-link

Improve event note display in dashboard and project activity views