Adapt the migrator to the change of max page names in 13.4

[snazare]

For versions of XWiki supporting 756 Characters (TODO: find method to check this), we should adapt some behavior of the importer:

1/ Confluence API check should not complain about pages being too close to 256 character as this won't be a problem anymore
2/ The tool to reduce page names should use a longer limit (currently 30 if I'm correct). This limit could be a setting in the import configuration so that a manual choice is possible. When not set at all we use 30 for older instances and 50 for more recent ones.
Note: currently in order not to fail the import we need 50 x N < 768, with N being the number of levels in the page tree.
So if we choose 50 than we can have almost 15 levels.
3/ Nested Pages Migrator should also use 756 to check any issues at that step (this setting is in Javascript at this point).

[acotiuga]

3/ Nested Pages Migrator was improved in https://jira.xwiki.org/browse/NPMIG-56 and the new version is used in the Confluence Migrator (27862da).