Html fix phase1

Introduction of the htmltemplate ServerSetttings concept to allow use of other templates than the current default.

This is to allow us to incrementally experiment with and then deploy a new default HTML presentation. There are three major changes to template handling and one change to `/tiddlers` globally:

* htmltemplate key for ServerSettings, currently defaulting to ''. If set the value is used as a subdirectory in which to search for the currently requested template. If it is not found there, use the template in the top level directory.
* If htmltemplate is set, then only use the HtmlCss and HtmlJavascript mechanism with the following URIs:
    * `/tiddlers`
    * `/search`
    * `/hsearch`
    * `/<friendly>`
* For those same URIs, if HtmlCss is not being used, then a `<htmltemplate>`.css tiddler is looked for in the `common` bag.

`/tiddlers` globally is changed to only show those tiddlers which are not in the core bags (that is the actual content of the space, not the content that is in every space), and they are sorted by modified time.

In its current state the `clean1` templates are incomplete but allow for experimentation and feedback.

Making this a pull request to get some review from @pads and @colmjude since I wrote the code, needs some looking over by others.