Create variable for subtitle for page title/header block

[markconroy]

LocalGov Core now has a {{ subtitle }} variable. Let's add the same to localgov_base.

Note: this will do nothing until a theme implements an event subscriber to set a field as the subtitle field. We are only putting the variable in position so we have it for themes that might use it.

Further details:

• https://github.com/localgovdrupal/localgov_core/releases/tag/2.13.0
• Consider stacked h1 pattern for guides localgov_guides#122

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Thanks to Big Blue Door for sponsoring my time to work on this.