Deprecate length_is template filter in favor of length.

[Nick Pope]

The length_is template filter is a vestige from the days of the {% ifequal %} and {% ifnotequal %} days before {% if %} arrived with support for comparison with operators. Even the example in the documentation (see ​here) is poor: {{ value|length_is:"4" }} will only return one of three possible values - True, False, or "", the empty string being for errors in the provided values.

It seems to me that it would be better to encourage use of the length template filter with the {% if %} template tag which can provide more flexibility:

{# Before: #}
{% if value|length_is:"4" %}...{% endif %}
{{ value|length_is:"4" }} ← This is of dubious use given the restricted set of possible output values.

{# After: #}
{% if value|length == 4 %}...{% endif %}
{% if value|length == 4 %}True{% else %}False{% endif %} ← Much clearer but also allows customising the output values.