[UI] Add configurability to the menu, what root view to show and what name to use

[nhoening]

Let the root view ('/') and menu be configurable by which account role(s) the current user's account has.
Let menu titles and icons be configurable.

This gives more control for supporting different kinds of services on one server. We introduce account roles for this, so that accounts can be treated differently.

To test this, one could play with the relevant config variables:

FLEXMEASURES_ROOT_VIEW = [("details", ["MDC"],), "assets"]
FLEXMEASURES_MENU_LISTED_VIEWS = ["dashboard", "details", ("clusters", ["MDC"])]
FLEXMEASURES_MENU_LISTED_VIEW_ICONS = {"details": "snowflake-o"}
FLEXMEASURES_MENU_LISTED_VIEW_TITLES = {"details": "DEAILS", "dashboard": "DASHBOARD"}

But first, a database upgrade:

flexmeasures db upgrade

Also, you can assign account roles to your user, e.g.

insert into roles_accounts (role_id, account_id) VALUES (2,1);

Alternatively, making a fresh account now can create account roles alongside:

flexmeasures add account --name "Some company" --roles MDC,Prosumer

[Flix6x]

Minor comments.

[nhoening]

I just noticed that there is another setting (FLEXMEASURES_PLATFORM_NAME) which we might want to treat much the same as the two settings I already worked on

[nhoening]

We also still need CLI command for adding a new account role. Deleting a role can be tricky if accounts are connected to it.

[Flix6x]

I reverted 164 so this can be tackled in a separate PR to main.

Just 1 more comment (to fix a docstring or the delete_account_role function's logic, according to whatever you had in mind), otherwise this PR has my approval.