Make sure all Infusion core demo index pages pass markup validation

Several of them currently don't, FE:
https://github.com/fluid-project/infusion/blob/master/demos/inlineEdit/index.html#L55
Should be "</figure>".

There are also markup issues in some other demos linked from http://build.fluidproject.org/ that exist in separate repos, such as http://build.fluidproject.org/prefsEditors/demos/explorationTool/ - they aren't being tackled by this PR.

Additionally, some Javascript components created for the demos have problems with their markup - see FLUID-5994 as one example. These should be corrected separately, since they're somewhat more complex cases.

Notably, the "check serialized DOM" bookmarklet at https://validator.w3.org/nu/about.html will show problems with the markup generated by Javascript components; putting in the page URL at https://validator.w3.org/nu/ will show only the markup as initially delivered by the server (thanks jobara for helping figure this out...)