UI enhance and dynamic pixel-rate

[xinze-zheng]

Overview

This PR enhances the frontend UI and exposes a route for pixel-rate change.
Acknowledgement to zoezheng126 with UX idea and help.
#11 #5

UI enhancement

• Adding alert when secret is invalid, missing, or color is not selected.
• Button will change and secret text field will be disabled when secret is accepted.
• Alert can be closed or will automatically close in 1.2 seconds.
  

Pixel_rate_change

• Expose a route to change pixel rate as shown below.
• Modify the update_pixel method that will return pixel_rate to PG everytime so that PG can be aware of pixel rate change. This feature might be implemented more decently with Socket.

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POST /changePixelRate

This route is used to update the pixel rate.

Request Data: JSON containing the keys "new_rate" and "token".

Response Status Codes: If everything is okay and the pixel_rate is updated, 200 OK is returned. If the response has an invalid token (i.e. the PG isn't registered), 401 Unauthorized is returned. If there are missing attributes in the json file 400 Missing attributes is returned.