Preserve context in postback payload & JWT

[MrOrz]

Under some scenario, user's manipulation will have unexpected results.

For instance, when the user is asked feedback from 2 replies in one search session, the user may want to give feedback to the former reply, but currently the chatbot always targets the last reply being displayed.

The root cause is the context (selectedArticleId, selectedReplyId) is not preserved when the postback or LIFF is invoked. If we preserve all context in JWT (for LIFF) or postback payload, we will be able to recover the context whenever the LIFF or postback is invoked.

To achieve this, the context should store as few data as possible. We should get rid of data.selectedArticleText and always read from DB (using selectedArticleId) instead. As for data.searchedText, the data is required to preserve in context, but we don't need to put it in JWT nor postback payload, since data.searchedText never change within one search session.

This picks up what is left behind after implementation of #49 .

[MrOrz]

Will address in #310