Change position of page titles and add page headers

[danieldegroot2]

For at least the following pages, add

• an offset to their page title. The second one could instead be positioned above the existing content.
  Offset is referring to the offset of i.e. event page titles.
  Positioning above existing content is referring to the position of i.e. the Documentation page title.
• a page header (example code given below).

'Join Event' on the Attend page. (code)

{% block title %}
    Join Event | OpenStreetMap Calendar
{% endblock %}

---

"New Event" on the Repeat Event page. (code - display code)

{% block title %}
    {% if page_title %}{{ page_title }} | {% endif %}OpenStreetMap Calendar
{% endblock %}

(similar could be done for Add Event page.)

---

Documentation is currently a single title "OpenStreetMap Calendar Manual", it could be split as above to assure the user they're still on the same website.

[thomersch]

Yes, they are not very consistent. I think this is a result of some stylesheet changes that I made but haven't check on all the pages if they're still ok.

The general idea is that almost all pages have narrow, center content, because it's easier to read, but there are a few which use a multi-column design (like the event detail pages) and fallback to single column on narrow devices. The titles themselves should be offset as you say as on e.g. the documentation page.