Provide a set of simple flamingo color themes in the standard distribution and move bootswatch themes to an extension

Currently, the color themes that are provided with the standard flavor of XWiki are the themes from bootswatch.

These themes are interesting, but they may be too much, if the only objective of the user is to just use a different main color between subwikis to distinguish them one from the other.

Also, one issue is that a regular user (that is not a developer) cannot obtain any result comparable to those color themes by just editing a color theme.
Also, for some of them the preview is not accurate (probably also because of them being written using advanced LESS / CSS). Also, there is XWIKI-14995.
They can also endup creating some issues for certain applications (such as contrast issues), if they target and style elements that are beyond the elements that are supposed to be styled by the Flamingo themes application.

I think that by default XWiki should provide some color themes that are just different color variations of the same base color theme (like the 4 color themes were provided for the colibri skin), built exclusively with the flamingo themes editor, and the Bootswatch themes should be provided as extensions, either alltogether or one by one.