Provide a wiki picker for macro parameters

Some macros take a list of wikis, and the Edit Macro dialog shows it as a plain text field: the user has to know the identifier of each wiki, which the UI rarely shows, and a typo is accepted silently. XWiki has pickers for a page, an attachment and a user, but none for a wiki.

Where it is needed

Proposal

• Add a #wikiPicker Velocity macro, which suggests the wikis of the farm and takes several values, and an HTML displayer edit template for List<WikiReference> that uses it.
• Declare List<WikiReference> as the type of the parameter above, so that the Edit Macro dialog shows the picker, and turn the typed value back into the comma-separated list that the macro expects.

An implementation is proposed in PR #5999 of XWIKI-23834, for the notifications macro. The activity macro is left out: it is a legacy macro, to be deprecated in XWIKI-25515.

Impact found on macro fluidity

This KO comes from the WYSIWYG usability audit of the macros, under the criterion C4 input-assistance of its usability criteria. The page of the macro, linked in the table, has the full notes.