* minor improvements

* REL info and asserts changes as per guidance.

* More consistent prefix.

[Misc] Since we use a third-party connection pool (DBCP by default) we don't need to set Hibernat's internal pool configuration (they're not used). See https://docs.jboss.org/hibernate/orm/3.3/reference/en-US/html/session-configuration.html#configuration-hibernatejdbc for example.

XWIKI-12884: No way to specify a webjars URL for a resource located in another wiki
XWIKI-12878: Webjar installed by EM on a subwiki could not be loaded through the webjar action

[Misc] Removed leftover from XWIKI-12045 ("Remove xwiki-platform-legacy-crypto-script module"). Thanks to Denis for noticing it.

* also add a missing older parameter

* Rename "useLegacyParentChildMechanism" to "hierarchyMode".
* Stop pretending parent child mechanism is deprecated. Is is just off by default.

* This commit introduces:
** a "core.useLegacyParentChildMechanism" property to use or not the hierarchy based on legacy parent/child relationship
** a "parentchild" script service to get the legacy parents of a document and to get if the legacy mechanism is used or not
** the support of both modes in flamingo
** use the home icon for the Main space
* This first version does not hide some parts of the breadcrumb if it is too long
* This commits does not fix the failing functional tests