Add render_uncached_placeholder templatetag

[yakky]

This templatetag is a version of render_placeholder which does not use cache, in a similar way to show_placeholder, show_uncached_placeholder

[yakky]

#3702 rebased on develop

[coveralls]

Coverage increased (+0.01%) when pulling 74e4c4e on yakky:feature/render_placeholder_uncached into 2ef8998 on divio:develop.

[yakky]

Ping @ojii @mkoistinen @digi604 this fill in a gap in the api

[mkoistinen]

cached or taken from the cache

[mkoistinen]

@yakky, it is unclear to me what should happen if someone adds a plugin with cache=True to a placeholder that is rendered with either show_uncached_placeholder or render_uncached_placeholder. What is the correct behaviour? Also, once we decide this, can we add some tests to protect the decided upon behaviour? I can see this getting flip-flopped a lot over time.

Otherwise LGTM

[yakky]

Caching is only at placeholder level, and cache = True only means: "Ok placeholder, I can be cached if you're up to".
So using show_uncached_placeholder / render_uncached_placeholder effectively disables any caching (both read and write), ignoring plugins attribute.
The current test coverage should already address this

[coveralls]

Coverage increased (+0.01%) to 90.96% when pulling 2eaf0bd on yakky:feature/render_placeholder_uncached into 4560601 on divio:develop.

[coveralls]

Coverage increased (+0.01%) to 90.96% when pulling 2eaf0bd on yakky:feature/render_placeholder_uncached into 4560601 on divio:develop.

[coveralls]

Coverage decreased (-0.09%) to 90.431% when pulling 74e4c4e on yakky:feature/render_placeholder_uncached into 2ef8998 on divio:develop.

[yakky]

@mkoistinen task completed

[mkoistinen]

lgtm