UX: Add a help modal dialog (PR #109)

This PR is based on this closed PR.

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I wonder if the modal content should just be one translatable string instead?

Yeah, it’ll definitely make things simpler.

@ZogStriP I wonder if it’s all right to put that much HTML in a translatable string when putting all modal content into it. The modal even contains a table:

Fait point. Maybe extract the table into another string or split the whole string in a few others?

I’ve split the modal content into 4 pieces and eventually removed the table and started using a list instead. It’s a bit tricky business to use a table here, we need a CSS class to be applied to the table, so we can’t put the whole table into a resource. With the list it looks like this:

Additionally, several parameter types accept easily paste-able text formats or provide lookup assistance.

Here is the full list of parameter types, which we do need to document somewhere: discourse-data-explorer/plugin.rb at main · discourse/discourse-data-explorer · GitHub