Comment visibility (#364)

Add some support for comment visibility, fix #217 

This add a new column to comment, denoting if they are public or not, and a new table linking private comments to those allowed to read them. There is currently no way to write a private comment from Plume.
Git is having a hard time what happened in Comment::from_activity, but most of it is just re-indentation because a new block was needed to please the borrow checker. I've marked with comments where things actually changed.
At this point only mentioned users can see private comments, even when posted as "follower only" or equivalent.

What should we do when someone isn't allowed to see a comment? Hide the whole thread, or just the comment? If hiding just the comment, should we mark there is a comment one can't see, but answers they can, or put other comments like if they answered to the same comment the hidden one do?

migrations/sqlite/2018-12-17-221135_comment_visibility/down.sql

@@ -0,0 +1,28 @@
+-- This file should undo anything in `up.sql`
+CREATE TABLE comments2 (
+    id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
+    content TEXT NOT NULL DEFAULT '',
+    in_response_to_id INTEGER REFERENCES comments(id),
+    post_id INTEGER REFERENCES posts(id) ON DELETE CASCADE NOT NULL,
+    author_id INTEGER REFERENCES users(id) ON DELETE CASCADE NOT NULL,
+    creation_date DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    ap_url VARCHAR,
+    sensitive BOOLEAN NOT NULL DEFAULT 'f',
+    spoiler_text TEXT NOT NULL DEFAULT ''
+);
+
+INSERT INTO comments2 SELECT
+        id,
+        content,
+        in_response_to_id,
+        post_id,
+        author_id,
+        creation_date,
+        ap_url,
+        sensitive,
+        spoiler_text
+    FROM comments;
+DROP TABLE comments;
+ALTER TABLE comments2 RENAME TO comments;
+
+DROP TABLE comment_seers;

migrations/sqlite/2018-12-17-221135_comment_visibility/up.sql

@@ -0,0 +1,9 @@
+-- Your SQL goes here
+ALTER TABLE comments ADD public_visibility BOOLEAN NOT NULL DEFAULT 't';
+
+CREATE TABLE comment_seers (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    comment_id INTEGER REFERENCES comments(id) ON DELETE CASCADE NOT NULL,
+    user_id INTEGER REFERENCES users(id) ON DELETE CASCADE NOT NULL,
+    UNIQUE (comment_id, user_id)
+)