Prompt for Mapbox token if needed

[aparlato]

Closes https://github.com/stamen/chartographer/issues/57

Prompts for a Mapbox token if needed for the typography chart.

QA

A

• [ ] Use style with Mapbox URLs for source, sprite, and/or glyphs (I just used Mapbox Streets)
• [ ] Switch to typography chart
• [ ] Expect to be prompted for Mapbox token
• [ ] Enter incorrect token without pk. at beginning
• [ ] Expect warning that tokens must start with pk.
• [ ] When prompted again, enter incorrect token with pk. at beginning
• [ ] Expect warning that token is invalid
• [ ] When prompted again, cancel the box
• [ ] Expect warning and redirect to fill page
• [ ] Click typography page again
• [ ] When prompted again enter correct token
• [ ] Expect that typography chart works
• [ ] Expect that renderer dropdown only allows Mapbox due to Mapbox URLs
• [ ] Change or clear style
• [ ] Add back previous style or another style requiring a token
• [ ] Switch back to typography chart
• [ ] Expect no prompt since you've added a token in this session

B

• [ ] Add a style that does not require a Mapbox token
• [ ] Expect no prompt

[aparlato]

One potential enhancement (follow up): what do you think about finding the access token if it's included as a parameter in the style URL? I used the URL of a Mapbox style from a Mapbox account that included access_token but was still prompted for it, seems like it'd be nice to simplify this for the user?

Good call! Will open a followup ticket to address this.