Enhancement: improve letters tier by always proceeding to the next target letter

[jdestgermain]

Is your feature request related to a problem? Please describe.
New users are often getting confused on the letters tier. The main problem seems to stem related to a variety of bugs/inconsistencies in how the letters are shown and how errors are handled.

One suggestion would be to get rid of the requirement to type the right letter to proceed to the next letter. Right now, it puts grey letters underneath the target letters and is not obvious to users. Space is not needed, but pressing space turns the target letter red and then let's you proceed forward.

I think we should instead make it so that space and backspace aren't needed at all (also see #64 ).

The default behavior should be that if you type the wrong letter, it proceeds to the next target letter and turns the letter that you didn't successfully type correctly red.

This way, the user can always make progress and instead never gets stuck.

It would look like this:

Instead of looking like this (which is often missed by users)

[jdestgermain]

Related issue that causes confusion #114