Feat: multiple themes

SuCoS now allow sites to install multliple themes (but still only use one). They will be placed inside a themes/ folder (or overwriting it using ThemeDir variable in the site settings sucus.yaml). It will also allow using nested git repositories and splitting the theme into other remote repositories.

BREAKING CHANGE: until now, the theme data was supposed to be placed in theme/ folder. Now, one must create a themes/ folder and put the theme data into a folder within it, like themes/my-theme