Distribute spacemacs-theme with Spacemacs so we don't need to download the
package of the theme at startup. It was delaying the display of the home buffer.
Now Spacemacs fallback to spacemacs-dark theme if the user theme cannot be
applied. Spacemacs then tries to install and reapply the user theme. If
successful, at the subsequent startups the user theme is applied right away
instead of spacemacs-dark. If the installation failed then we display a warning
informing the user and suggesting some actions.
There is now no package left to be installed manually at the start of Spacemacs.
