attachment内の寿司に対応

[shundroid]

Closes #186 .

リンクのプレビューなど、後で追加された attachment も対応

[codecov[bot]]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (44aadc6) 23.30% compared to head (e69f37a) 23.28%.

:exclamation: Current head e69f37a differs from pull request most recent head 7a586f1. Consider uploading reports for the commit 7a586f1 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #854 +/- ## ========================================== - Coverage 23.30% 23.28% -0.02% ========================================== Files 174 174 Lines 15364 15372 +8 Branches 3514 3519 +5 ========================================== - Hits 3580 3579 -1 - Misses 11103 11112 +9 Partials 681 681 ``` | [Files](https://app.codecov.io/gh/tsg-ut/slackbot/pull/854?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tsg-ut) | Coverage Δ | | |---|---|---| | [sushi-bot/index.ts](https://app.codecov.io/gh/tsg-ut/slackbot/pull/854?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tsg-ut#diff-c3VzaGktYm90L2luZGV4LnRz) | `54.18% <78.94%> (+0.27%)` | :arrow_up: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/tsg-ut/slackbot/pull/854/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tsg-ut)

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[shundroid]

うおーたしかに、、、 リアクションは結局 message_changed の時に再計算しているので、いったん全部sushi-bot由来のリアクション外す、とかでもいいですかね 寿司ランキングは差分がわからないとですね、previous_message が message_changed でわかるので、それを使うことになりそうです

こういう方針で実装してみましょうか？ @hakatashi

[hakatashi]

よさそう!

[shundroid]

@hakatashi これすみません！直近時間が取れないので、もし可能であれば「とりあえずデプロイしてそのうち直す」でお願いしてもよろしいでしょうか。

[hakatashi]

よさそう! マージします