Webadmin improvements

- Added "files" folder.
- Created a login_template.html file
- Exported stylesheet in a own file
- Exported favicon
- Stop webadmin if template can't load.
Move this entire `else` branch into a separate method, `SendDiskFile(cHTTPConnection & a_Connection, const AString & a_URL)`. The function is already quite long, no need to make it even longer.