Linter: Detect exported functions/interfaces/etc without comments

We can enable the linter rule to require adding a comment to the exported functions/constants/types/interfaces/structs. I often forget to do it.

I prepared a patch below. To apply it, we need to fill in the missing comments.

diff --git a/backend/.golangci.yml b/backend/.golangci.yml
index 0f394c35..6240b22b 100644
--- a/backend/.golangci.yml
+++ b/backend/.golangci.yml
@@ -75,7 +75,8 @@ linters-settings:
 
 issues:
   max-same-issues: 3
-
+  include:
+    - EXC0012  # EXC0012 revive: Annoying issue about not having a comment.
   exclude-rules:
     # Exclude some linters from running on tests files.
     - path: _test\.go