[META] Remove dependency to Classy from core themes

We'll need some announcement around this. People are prone to copy Bartik and Seven, so it would be good to let them know this dependency on Classy is going away and they should do the same thing core is doing and copy its templates to their theme.

At the time I'm writing this the Core Theme's use of Classy has documented everything other than CSS files. There are two takeaways I wanted to share here as soon as possible.

1. 24 of the 113 templates in Classy are identical to their equivalents in Stable (reference the spreadsheet's "Templates" tab, look for the ones labeled "can use stable". It seems like these would not need to be copied to other themes, but since they account for 20% of the templates in Classy, I'm wondering if there's a reason for their presence that I'm overlooking.
2. Based on my info gathering, it seems like a good first step would be moving Classy's libraries. Since most (probably all) of these libraries are used by multiple themes, I think it would be most effective to move them to a centralized location that is available to all themes rather than duplicating the library in every theme. This boils down to two questions:
   a) Is it a sound approach to move Classy's libraries to a single new place where they are available to all themes?
   b) If it is a sound approach, would this be best implemented as core libraries, in Stable, or somewhere else?

Added the issue #3095713: Create classy directory with README, in the templates and css directories for all themes subtheming Classy, which is a prerequisite for any issues copying Classy templates to other themes.

Discussed my questions in #5 with @lauriii, this is what was concluded
#5.1: Templates that are identical in Classy and Stable do not need to be copied. These themes can inherit those templates from Stable.

#5.2: For the time being, each theme will get their own copies of the libraries they previously acquired via Classy. Centralizing these libraries is possibly desirable, but can be done in a followup.

Moving forward on this depends on these three issues:

• #3095713: Create classy directory with README, in the templates and css directories for all themes subtheming Classy determines where the Classy-copied templates go
• #3096349: Create test for confirming Themes do not depend on Classy templates creates tests that can used for all issues that require creating theme-specific copies of Classy templates
• #3096203: Create Classy library dependency tests that can be used for all themes, and verify by providing an Umami-specific classy/dropbutton creates tests that can be used for all issues that require making theme specific versions of Classy libraries

Created the issue #3098150: Add test coverage to ensure Classy templates that have identical versions in Stable are kept in sync for removing templates from Classy that have identical equivalents in stable and updated IS to reflect this.

Added theme-specific parent issues to issue summary.

Added #3108109: Decouple Classy libraries from Claro to issue summary.

Added #3108181: Decouple Classy libraries from Bartik to issue summary.

Added #3109287: Decouple Classy-inheriting themes from Classy's preprocess functions to issue summary.