Fixes #54 #63 - multi explores enhancement

[pjabl]

54

• multiple explores from the dropdown
• contextual history / samples (selected by explore_id)
• samples from BigQuery (by exlore_id), allowing technical team to set up different samples per explore_id and load together with examples

I've also moved within main README Frontend setup after Examples generation instructions link; the reaosn is, we need to have BigQuery populated before testing Frontend; for example, without explore_assistant_refinement_examples populated I got an error

Multi explore Select added to both the assistant and chat pages.

63

If target user choses to rebrand explore assistant to something else, certain headers / titles should be configurable.

[LukaFontanilla]

We've had a major UI/UX refresh. I took the core ideas in this PR and brought them into a new PR that works with the latest UI refresh of Explore Assistant. Thank you for the great multi explore idea!