Added "use ..." statements

This version ditches backwards compatibility with PHP earlier than 5.6.
Updated TODO
Split FileService to NetworkingService and FileDao
Reduced responsibilities of HttpHelper
Added meaningful name to saved posts
Faster ETag computation
Added random optimizations
Added ability to loop video posts
Added robots.txt
Removed PathJS dependency; refactored routing
Fixed avatars for anonymous users
Changed pager spinner to be smoother
Implemented mass tag
Added tag name changing
Improvements to appearance
Fixed frontend crashing when database is empty
Added default timezone
Fixed comment saving
Added migration script
Fixed cron script