Feat: search for comments within posts

Add the ability to search for specific comments within posts.
Known issues:
  - Just like on reddit, this does not work with comment sorting. The
    sorting order is ignored during the search and changing the sorting
    order after the search does not change anything. I do not think we
    can fix this before reddit does, since in my understanding we rely
    on them for the sorting. However we could implement a default
    sorting method ourselves by taking the vector of comments returned
    from the search and sorting it manually.
  - The UI could be improved on mobile. On screens with a max width
    inferior to 480 pixels, the comment search bar is displayed below
    the comment sorting form. It would be great if we could make the
    search bar have the same width as the whole comment sorting form
    but I do not have the willpower to write any more css.

build.rs

@@ -1,6 +1,4 @@
-use std::{
-	process::{Command, ExitStatus, Output},
-};
+use std::process::{Command, ExitStatus, Output};
 
 #[cfg(not(target_os = "windows"))]
 use std::os::unix::process::ExitStatusExt;