Add ability to mark models as embedded and then place them with placeholders

Right now, it is assumed that all bokeh models are placed together in a single place on the html form. If you want to separate them with intervening text, for example, we should allow:

- Marking a model as 'relocatable' that tags it to not be added to the set of models in the middle of the html output.
- Processing the contents of before_content and after_content for a placeholder specified in the call to `relocatable_model` and putting the model div in the right place there.
- Making sure the fixed width sizing works correctly.