Xss::filterAdmin() to allow "template" elements

Problem/Motivation

Custom Text in Views do not allow rendering template tag, which is a powerful way to do Javascript rendering templates in HTML. The list of tags allowed is limited by Xss::filterAdmin (as specified in Xss::$adminTags).

Steps to reproduce

Create any view that renders template element.

Proposed resolution

Add 'template' to Xss::$adminTags.

Remaining tasks

Discussion and code implementations.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

Allow to render "dialog" element in Views "Global: Custom Text" and other rendering context using Xss::filterAdmin.