fix(demo): Various fixes related to schema validation

[flochtililoch]

Fix various issues related to schema validation, in particular due to our set up with 11ty:

• Front matter ending up in generated XML files - this happens with files that were both destined to be served from their own endpoints but also included from other templates. To solve this, I created intermediate files for those that need to be served, and remove the front matter for those that are just included.
• I've also updated the list of files in .eleventyignore to be explicit - this list represents all the templates that are meant to be included but should not be served as endpoints (hence not generated as static XML files). In the future, we will segregate private and public files to avoid the need to define this list.

Also in this PR:

• remove exclusions from the XML validation command - now most generated templates are validated (the templates related to the share functionality, which uses an instawork namespace, is still ignored for now, it will be ported to a custom behavior in a future PR)
• change the custom bottom bar component namepace prefix from instawork.com to hyperview.org
• fix a few templates that would produce invalid XML
• fix a few schema definitions that were incorrect