Total number of posts displayed on discussion list marked as read status is different from unread status.

## Bug report
- Version of Flarum: 0.1.0-beta.7
- The webserver you are running: apache, nginx
- PHP version: 5.6

## Flarum info

c:\phpDir\htdocs>php flarum info
Flarum core 0.1.0-beta.7
PHP 5.6.30
Loaded extensions: Core, bcmath, calendar, ctype, date, ereg, filter, ftp, hash, iconv, json, mcrypt, SPL, odbc, pcre, Reflection, session, standard, mysqlnd, tokenizer, zip, zlib, libxml, dom, PDO, openssl, SimpleXML, wddx, xml, xmlreader, xmlwriter, curl, fileinfo, gd, mbstring, mysql, Phar, pdo_mysql, PDO_ODBC, xmlrpc, mhash
EXT flarum-approval v0.1.0-beta.7
EXT flarum-bbcode v0.1.0-beta.5
EXT flarum-emoji v0.1.0-beta.6
EXT flarum-english v0.1.0-beta.7
EXT issyrocks12-filter 1.2.2
EXT flagrow-upload 0.5.7
EXT flarum-flags v0.1.0-beta.7
EXT flarum-likes v0.1.0-beta.6
EXT sijad-links 0.1.0-beta.6
EXT flarum-lock v0.1.0-beta.7
EXT flarum-markdown v0.1.0-beta.5
EXT s9e-mediaembed 0.3.2
EXT flarum-mentions v0.1.0-beta.7
EXT jsthon-simplified-chinese v0.1.0-beta.5
EXT flarum-sticky v0.1.0-beta.7
EXT flarum-subscriptions v0.1.0-beta.6
EXT flarum-suspend v0.1.0-beta.7
EXT flarum-tags v0.1.0-beta.8
EXT issyrocks12-users-list 0.2.2.1

## Additional comments
1.post a discussion
2.one person reply the discussion
3.admin user stick up the discussion
(Total number of posts displayed on discussion is 3)
4.mark all read
(Total number of posts displayed on discussion is 1)
The result made me confused. I think both number of posts are one.

Start by reproducing the numbered steps in the issue on Flarum 0.1.0-beta.7 and compare the discussion-list post count before and after marking all discussions as read. No source file or test is named; done means the displayed count is consistent for both read and unread status.

Written by the indexing model from the issue text.