Forbid newlines in tags

[tonnzor <tonn81@…>]

Newlines must be forbidden in tags. If tag contains newline (\n or \r) it should be processed as ordinary text.

Inspired by ​Templates: short comments {##} eats text

[Malcolm Tredinnick]

(In [5104]) Fixed #4164, #4171 -- Reworked some of the template lexer logic to ensure we
don't get caught out by a couple of corner cases.

[anonymous]

This doesn't work for templates that extend other templates. Simple test case:

A.html:

   {% extends "B.html" %}
   {#
   foo
   #}

B.html:

   <h1>hello</h1>

This incorrectly hides {# foo #}.

[Chris Beaven]

That doesn't show anything - {% extends %} only replaces parent {% block %}s with ones in the child template so it hides any text you'd put outside a block tag.