Add general topics

[mciethan]

fixes #46

instead of a single list of topics containing indicators, we now have "general topics" like demographics, housing, businesses, which becomes its own drop down menu, with specific topics contained within those general topics

• [x] make menu interactions functional

Based on feedback from Mandy, I'd like to try some additional steps:

• [ ] convert general topics drop-down into a multiselection of buttons
• [ ] unselecting a general topic should filter out its specific topics from the topic menu, but the general topic shouldn't disappear - ideally selected and unselected would be differentiated by bolds or etc, maybe by making a set of checkboxes

[mciethan]

Checkboxes are a decent idea, and R Shiny supports them (https://rdrr.io/cran/shiny/man/checkboxGroupInput.html). There are things to figure out, though. For one, making sure that having 0 checkboxes selected behaves in a reasonable way. That should either result in everything (charts, map, slider) being blank, or it should act like you've selected all the topics.

Also, for general topic parameters, you need some way of concatenating stuff across multiple general topics. That implies foreclosing the possibility of general topic parameters besides the topics, because how would you concatenate different parameters. I'm ok with that, and happy to embrace the general topic as more of a filtering condition than an nesting layer, as long as CONFIG and server are refactored to just have the list of topic names rather than having the additional list named "topics" that we currently do.

However, we could go farther in that direction and treat the general topics as tags which one topic could have multiple of. If that's even a remote possibility, then it's better to revert to topics as the outer structure and instead have a tags parameter for topics which we then collect upon app load in order to create a grouped checkbox at the top of the controls.

UI-wise, I think we can have "Filter topics by subject", "Choose a topic", and "Choose a variable".

I may end up creating a separate branch since the structure is going to be more similar to main than to this branch.

[mciethan]

Mandy and I decided that general topics should act like tags / filters (not like containers) of specific topics. There is therefore no need (and some harm in user experience) for having a new dropdown menu to select general topics first. Will start a different branch to implement general topics as a checklist that filters the specific topics menu.