Simplify state, return state only when we need it

[nonumpa]

Fixes #177

1. Remove useless state assignment and return state only when we need it

   • ASKING_REPLY_FEEDBACK state is replaced by input.startsWith(UPVOTE_PREFIX or DOWNVOTE_PREFIX).
   • In [handleInput], context.state is mainly used for postback message. But now it's also used to distinguish state ASKING_ARTICLE_SUBMISSION_CONSENT and ASKING_REPLY_REQUEST_REASON from input.startsWith(SOURCE_PREFIX). Maybe we can use different prefixes for them in the future.

2. Move reason message handler from [initState] to [askingReplyRequestReason]

   • Merge reason message(original in initState) to ASKING_REPLY_REQUEST_REASON.

3. I don't update test in this PR, so we can see if there's fatal test fail after this refactoring.
   Will fix test in another PR.

   • In [initState.test], all handles reason LIFF should test in [askingReplyRequestReason.test].
   • In [handleInput.test], the test processes first article reason submission is wrong, REASON_PREFIX was processed in [initState] not [askingArticleSubmissionConsent].
   • In [askingReplyRequestReason.test], because we merged reason message handler into [askingReplyRequestReason], the test should block incorrect prefix shouldn't input REASON_PREFIX now.

[MrOrz]

The code change looks reasonable and it works on my local machine / test accounts.

Thanks a million for simplifying the code base! Looking forward to the fix of test cases.