bbs_string, sticky_post, loading and others

Use bbs_string in config to replace specific strings appeared in other
js codes. add sticky post functions. Fixed comment typo. Change loading
icon to previous version.
sticky post list added to UI

still has bugs from pybbs that needs to be fixed. Color difference needs
to be added. Folder of wheat_land needs to be fixed. All global
variables are now added to bbs_global.js
sticky posts can be read now

fixing wheat_land unread sign bug. Add error msg when user try to click
next/prev/sp post on sticky posts.