New option that allows making completed/entered habits grey

Works the same way as hiding habits, but only changes the color.

The reason this is useful, is because when a habit is treated as completed/entered, the cause might be just the frequency set for that habit.

So, if it's once per month and you've entered it only once, till a month has passed the habit will be hidden, but the habit might've been done again within that month and it would be useful to be able to see it to enter that data.

At some point it'll be good to change the darkest grey in the habit palet in "Edit habit", so that it doesn't conflict with the archived habit and "grey complete" colors.

It'll be good to call finish() on the settings activity, to avoid stacking activities every time the setting is changed.

Closes: https://github.com/iSoron/uhabits/discussions/2103

Option disabled:

Option enabled: