Fix EZP-20509: cache issues when creating content

JIRA issue: https://jira.ez.no/browse/EZP-20509

Description
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After the creation of a content in the admin interface, if you previously visited the frontend, the new content does not appear in the demo design until you clear your browser cache (or disable it). This is mainly due to the generated etag and the last modified header which do not reflect the entire state of the generated page but only the state of the content addressed by the URL (plus some parameters).
The patch removes the etag (until we have [a proper etag generator](https://jira.ez.no/browse/EZP-20307)) and the last modified header to avoid this issue.

In addition, the DemoBundle controller actions generate HTTP responses that are cached for 1 minute. This is not very efficient and because of this, the newly created elements are not immediately visible if they are supposed to be displayed through one of those actions.

See https://github.com/ezsystems/DemoBundle/pull/3 for the DemoBundle patch

Test
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Manual tests