Implement edit interface for TemplateData documentation

This adds a TemplateData manager to the edit page in the Template
namespace.

If a <templatedata> tag already exists, the tool will parse it and
display it in a visual interface that the user can then manipulate in
their own language. If there is no <templatedata> tag, the dialog will
appear empty and allow users to add and tweak their desired template
parameter data.

The tool also allows rudimentary parameter import, which picks up the
parameters from a current template into the GUI to make the user's life
easier when producing TemplateData.

The template documentation editor feature is off by default. To enable it
use $wgTemplateDataUseGUI = true in LocalSettings.

Bug: 50436
Change-Id: I863a8199c0b08cc52b320ed00dcba912dd2aeefc
Merge "Implement edit interface for TemplateData documentation"