Use text-underline for entities

Refs https://gitlab.com/arkindex/requests/-/issues/597

This redesign lightly the entities display, to remove borders and blocks, relying only on text flow with text underline: we do not have any placement issues then.

Notable changes:

• SASS use with declarations of color instead of relying on JS style. General comment: It would be a lot better to have a variable.scss instead of applying style from vue.js
• I removed one extra level of recursion in the tokens usage: a Token was added for each entity to display itself. It was weird, and added an extra layer of spaghetti HTML which was hard to use (and broke the text-underline)
• I removed the cleanText as it seemed never used: the entity property was always null due to the extra recursion level.

Demo on an entity with long & short entities, various types, and nested entities