add custom startpage support

[empi89]

Introduced config variable DEFAULT_TEMPLATE to specify a path to a template shown as start page.

Splitted this from #56

Regarding questions of @zorun :

• Rendering of the layout.html template requires population of session.
• Using render_template works only when templates are within the source directory. However, I would like to have them in an external path separated from the source code.

[zorun]

Thanks for coming back after 6 months 👍

Now I better understand what you are trying to do:

• if you want to display a specific page by default, you should use a redirection
• if what you want is a custom landing page, you should not need the set_session part

That being said, it seems that the lg.js indeed needs request_type etc from the session. There are some hard-coded cases for request_type == "summary". I think this needs to be sorted out instead of hacking something random into the session state.

What you can try: simplify layout.html so that it only handles the HTML layout and nothing else. Move everything that handles user requests (request_type and so on) to a new template like request.html, and base all existing templates on this one.