feat: forum-like signatures in posts #67

the default, as stated in https://witchsky.app/profile/xan.lol/post/3mf3v54zok22o, could be - Made with Witchsky, shaving off 21 characters from the post character limit. By default this setting would be off. Users would be able to enter their own 'signature' to append to their posts, with an indicator next to that input that shows the total characters used.