Improve and simplify strength handling in text input

[satazor]

Description

There's logic around the strength indicator on text input that can be improved and simplified.

This task was created after this comment.

Acceptance Criteria

• [ ] Remove any level compute logic from <TextInput> and move to <StrengthIndicator>
• [ ] Remove levelName and numberOfLevels props from <StrengthIndicator>
• [ ] Create a strength-colors.js and implement getColorForStrength, to be used by both components
• [ ] Remove onColorChange from <StrengthIndicator> as it's no longer necessary
• [ ] Remove strength level css colors (weak, poor, etc) and use style instead
• [ ] Check if making <StrengthIndicator> a functional component with hooks would further simplify it