improve theme application

[rob-metalinkage]

ideally theme customisation should be by configuring the base to find the theme - not copying any content into the local base clone. this will make development much easier too and allow regression testing against multiple themes

[nicholascar]

At the moment, VocPrez uses 17 templates, some CSS and images and the OGC theme is overwriting just 3 templates, the CSS file and 1 image.

If I allow VP's theme directory to be configurable, then implements of themes, like this OGC one, will need to supply all 17 templates, rather than just the bits the want to override, so I assume this isn't what you want.

I assume what you want is for VP to have the plain templates there but then to be configurable to pick up CSS, images and override templates without cop 'n paste override. I can't work out how to do this. CSS and some images are all referenced from the base HTML template, to this could be configurable (pointed to any dir) and save that copying but there seems to be no way in Flask (Jinja2) to draw templates from more than one location, e.g. defaults and overrides.

[nicholascar]

The only Flask way I can see of allowing multiple template locations is to use a Flask Blueprint per endpoint so any may be overridden. This will have the effect of requiring more file overrides.

For now (~2.x releases of VP), I'm going to leave this open but marked wontfix.