Official theme support

We already have the basis of a framework for 3rd party themes. But I think a good step would be actually trying to create an example 3rd party theme, documenting how this is done, and making whatever changes would be necessary in the cms to support this. The first example could be simple, still based on Bootstrap. It is possible to build a theme that swaps out bootstrap for something like foundation, materialize, skeleton, etc., but that is a task for a different day.

Requirements for the theme would be:
* Custom templates that override coderedcms templates
* Custom page and block templates in addition to coderedcms templates, utilizing the Layout or Adv Settings > Template chooser dropdown.
* Custom css and js added to base.html, to show how to properly do that.
* MAYBE some extra blocks or models that are part of the theme, to show how to integrate those into the cms as well.

The theme itself should be a separate pip package. But docs and any enhancements to support theming should be integrated into the CMS.

Theme support is currently possible, but is partially blocked by issues #8 and #56

首先审查现有的第三方主题 framework，以及 issues #8 和 #56 中的 blocker。使用 base.html 和 Layout 或 Adv Settings > Template 中的 Template chooser 作为指定的入口点，然后验证单独的 pip package 能够演示自定义模板、CSS、JavaScript 以及任何额外的 block 或 model，同时集成 CMS 文档和增强功能。

由索引模型根据 Issue 内容生成。