Create unified breadcrumb module

[joewiz]

As noted in #117 and demonstrated in #141, a unified breadcrumb module would help with maintenance and predictability.

Problems with current approach:

• Each page that needs breadcrumbs uses a combination of hard-coded and dynamically generated breadcrumbs (i.e., breadcrumbs generated by referencing templating functions)
• Hard-coded breadcrumbs are repetitive across pages and typo-prone
• Dynamic breadcrumb functions vary
• Moving a page or section requires manual tweaks to each page's breadcrumbs

Ideally, breadcrumbs would be:

• Centrally defined in $config:PUBLICATIONS
• Modular - allowing a deeper URL to reference breadcrumbs for its parent/ancestor pages

Examples of current approach:

• hard coded: https://github.com/HistoryAtState/hsg-shell/blob/master/pages/historicaldocuments/index.xml#L5-L12
• dynamic:
  • https://github.com/HistoryAtState/hsg-shell/blob/master/modules/frus-html.xqm#L307-L347
  • https://github.com/HistoryAtState/hsg-shell/blob/master/modules/tags-html.xqm#L70-L74

[joewiz]

Similarly, as noted in #161, sidebars are hardcoded. We should consider sidebars as part of this too - whether our approach is via includes or modules, etc.