Pkgdown config file in template packages

[maelle]

It would be nice if pkgdown template packages had the ability to contain a configuration file, that'd be combined with the package configuration file.

Elements that would be shared via the configuration file in a template package:

• Bootstrap variables after Switch to bootstrap 4 #1409 is tackled;
• Authors definition;
• Sidebar configuration (and footer configuration when that becomes flexible as well);
• Other variables see e.g. the non template variables mentioned in quillt https://github.com/apreshill/quillt/#using-quillt

[maelle]

Google Analytics tag ID

[maelle]

A link for getting help

[maelle]

Some error messages mention the config file as origin of the errors. When the values pkgdown actually uses are a mix of config in template package + config file + overrides, things might be confusing? So hopefully template packages wouldn't introduce errors (e.g. a non-inline element the html of an author).