Revert "Rename SITESUBTITLE to SITE_SUBTITLE"

This reverts commit e479ce72db916e7e21bc6ae63677221c8e290840.

As [@calfzhou](https://github.com/calfzhou) [pointed
out](https://github.com/talha131/pelican-elegant/commit/e479ce72db916e7e21bc6ae63677221c8e290840#commitcomment-6284503)

> In pelican's document, the default theme uses SITESUBTITLE.
Update thanks file
Fix a bug which will cause site name and menu to move to left on wide displays
Update release notes
Merge branch 'master' into calf