Frontend bug fixing

[IlinValery]

• [x] Key - make the field with a button, to show/hide text @IlinValery
• [x] Check, if each field is not empty for encode/decode @IlinValery
• [x] Change faveicon @PDementevna
• [x] Add button for href encrypt/decrypt @IlinValery
• [x] Allert on the main page - this project is only for edu @PDementevna + add a timer when the server will be turned off (from @IlinValery) :)