Enable plugins to decide which view is shown after logging in.

[create-issue-branch]

closes #100

[Flix6x]

I like it, but it wasn't obvious to me that the PR title reflects its contents. As is, should this really close #100?

Maybe consider adding an example that actually overrides the "/" route. Just adding @flexmeasures_ui.route("/") before an existing (plugin) route should do the trick, right? Theoretically, could one set one of the other existing FM routes to become the default after logging in, for example, the portfolio view? Perhaps by doing this in the plugin:

from flexmeasures.ui.views.portfolio import portfolio_view

@flexmeasures_ui.route("/")
def default_portfolio_view():
    portfolio_view()

[nhoening]

Yes I believe this PR could have a different title, like "Plugins can override existing routes", and one consequence is that they could rewrite "/", which is usually where you'll go after logging in (you go to where you wanted to go before you were rerouted to the login form, and that usually is "/").