Add support for custom scales

[ghost91-]

User Story

As a user of Next Generation Scrum Poker, I want to use my team's custom scale to estimate so that I don't have to translate between the estimations made in Next Generation Scrum Poker and my team's scale.

Context

In our team, we use a custom scale for estimating stories. Our team is called "Cookies", so obviously our estimation scale is also based on cookies. The scale is

• "Crumbs"
• "Bite"
• "Whole"
• "Plate"
• "Jar"
• "Cookie Cake"

Obviously, this scale is much too specific to be included in Next Generation Scrum Poker directly. For that reason, I would like to be able to manually specify a custom scale instead (e.g. as a comma separated list of values).

I already created a proof of concept implementation, which basically just adds an input box below the "Change Scale" select element that includes a comma separated list of the card values of the currently used scale. It is automatically updated when one of the predefined scales is chosen but it is also possible to change it manually:

It should probably be made a bit more beautiful but it already demonstrates how this could work.

The proof of concept implementation can be found at https://github.com/ghost91-/next-generation-scrum-poker/tree/custom-scales-feature. I already created a PR (#69), so the discussion can also be continued there.