Use config entities for styles

Problem/Motivation

There are few disadvantages to keep styles as a sequence in config object. Make them entities instead.

Proposed resolution

In order to achieve that, a few other changes need to happen. This issue will gather all of them, commit-by-commit.

Remaining tasks

Update and migrate paths, as well as the cookie update path, will be provided at the end of the development of this feature.

User interface changes

• Changed a few translatable strings
• Removed custom-style and theme-style HTML classes

API changes

• Removed the $type argument in the styleswitcher_style_load() function

Data model changes

• Replaced the styleswitcher.custom_styles config with the styleswitcher_style config entities
• Changed style name property to id

Release notes snippet

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