Fix/16483 progress bar was broken

* re-implements the progress bar to
** not use a table-layout (in angular as well as in the non-angular pages)
** not have box-elements inside of inline-elements in the HTML
* fixes: https://www.openproject.org/work_packages/16483
* documents `progress_bar` helper
* fixes flaws in the roadmap view
** the percent-sign was drawn twice in the progress bar legend
** `start_date` and `due_date` are only printed now when they have a value (so we have no empty labels confusing the user)
* renames occurences of the word `pourcent` (which is frensh) to 'percent'
@tessi can you move this out of this file and under `app/assets/stylesheets/content` instead?