Note: this task is reliant on {T260765}. Three of the four parts below are possible without, but the header design is based on the one to be implemented in {T260888}.
**Requirements**
This ticket can be broken into four parts:
[] Add the possibility to mark templates as deprecated, add guidance to TemplateData.
[] Add the possibility to mark and fill in guidance in the TemplateData editor.
[] Create a custom header for deprecated templates, including a warning, in the VE template dialog.
[] Templates marked as deprecated do not display in the list on the initial search dropdown to insert templates. If a user types the exact name of a template anyway, then they can add it. (This is the same current behavior as deprecated parameters.) The warning header will also be shown on existing templates, which later get marked as deprecated after they've already been inserted.
**Mocks**
TemplateData editor: add checkbox
{F32188698}
TemplateData editor: checked, with guidance input below
{F32188710}
VE template dialog: header with warning
Background: Yellow90 #FEF6E7 Outline: Yellow50 #FFCC33
Warning use: MessageWidget (type: 'warning', inline: true)
{F32188754}
Specs:
{F32188741}