squashed a whole bunch of PRs

closes #1907
closes #1908
closes #1909
closes #1910
closes #1911
closes #1912
closes #1913
closes #1914
closes #1915
closes #1916
closes #1917
closes #1918
closes #1919
closes #1920
closes #1921
closes #1922

The documentation says sessionIDs are between 22 and 256 chars long. A
quick test only showed 26 chars in common configurations, but this
should cover all possibilities.

When an invalid session ID is passed to PHP a warning is thrown, but the
session is still initialized with this invalid ID (throwing additional
warnings on save).

This makes sure such invalid IDs are removed from the cookie array
before initializing the session.

PHP bug references:

https://bugs.php.net/bug.php?id=68063
https://bugs.php.net/bug.php?id=73860

This partly fixes #1883

direction is always ltr for russian

PHP 5.6 and below throw a strict standards warning at the changed lines.
An intermediate variable is introduced to avoid this warning.

PHP 7+ changes the severity of this warning to E_NOTICE which is
suppressed by DokuWiki.

This error was introduced in #1778

similar to what is possible for plugins, templates can now autoload
namespaced classes, too. the namespace is
dokuwiki\template\<templatename>

This makes it more flexible to use on the expense of needing one echo
more.

$pagelog->getRevisions(-1, 1) cannot be passed to array_pop() as the
parameter $array expects a reference.

All script tags are now surrounded by conditional comments to tell
Internet Explorer 8 and lower to not even bother trying to load the
JavaScript.