This commit adds a {% absolute_site_logo_url %} template tag.
The tag emits an absolute url pointing to either
SITE_LOGO_URL or to the fallback picture.
The tag is used in base email template, in slack message
template, and in "Add MS Teams" page.
This commit also fixes a couple instances where absolute URLs
were constructed like so:
{% site_root %}/docs/
This would result in incorrect links if Healthchecks is not
running at webserver's root. The correct way is:
{% site_root %}{% url 'hc-docs' %}
Finally, this commit removes stuff/logo.svg and
stuff/logo-full.svg. Selfhosted sites should not use the
official Healthchecks.io logos, so no point keeping them around
there.
