Currently we have two kinds of user documentation:

* the about page[1] which is written in HTML and part of the web
  application and can therefore link instance-specific pages
  (like e.g. the preferences) via Jinja variables

* the Sphinx documentation[2] which is written in reStructuredText
  and cannot link instance-specific pages since it doesn't know
  which instance the user is using

The plan is to integrate the user documentation currently in Sphinx
into the application, so that it can also link instance specific pages.
We also want to enable the user documentation to be translated.

This commit implements the first step in this endeavor (see #722).

[1]: searx/templates/__common__/about.html
[2]: docs/user/ (currently served at https://docs.searxng.org/user/)

This commit remove the need to update the brand for GIT_URL and GIT_BRANCH:
there are read from the git repository.

It is possible to call python -m searx.version freeze to freeze the current version.
Useful when the code is installed outside git (distro package, docker, etc...)

In the templates and the /config (JSON) the usage of the 'brand.*' name
space is replaced by 'searx.get_setting' function.

- new_issue_url          --> get_setting('brand.new_issue_url')
- brand.GIT_URL          --> get_setting('brand.git_url')
- brand.PUBLIC_INSTANCES --> get_setting('brand.public_instances')
- brand.DOCS_URL         --> get_setting('brand.docs_url')
- brand.ISSUE_URL        --> get_setting('brand.issue_url')
- brand.CONTACT_URL      --> get_setting('general.contact_url', '')

The macro 'new_issue' from searx/templates/*/messages/no_results.html
is now imported with context::

    {% from '__common__/new_issue.html' import new_issue with context %}

To get *public instances URL* from context's 'get_setting()' function::

    get_setting('brand.public_instances','')

Macro's prototype does no longer need the 'new_issue_url' argument and has been
changed to::

    macro new_issue(engine_name, engine_reliability)

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

[fix] migration from github.com/asciimoo/searx to github.com/searx/searx : fix URLs