LIFF <> GraphQL server communication

[MrOrz]

This PR:

• Adds src/liff/pages/{Source|Reason|PositiveFeedback|NegativeFeedback}.svelte page stubs
• Implements loading state in src/liff/index.html and src/liff/App.svelte
• Implements gql in LIFF that sends authorization header, and graphql server that verifies the authorization header
• If session ID is not valid, tell user that the button is expired and aborts LIFF
• Initializes LIFF and handle the weird redirection mechanism of LIFF v2
• Add LINE user id in LIFF JWT tokens
• Removes old context JSON API; replace issuedAt with data.sessionId in GraphQL context API.

Screenshot

LIFF button opened previously

If LIFF button is pressed before, pressing it again could be pretty fast:
https://drive.google.com/file/d/1hpv2Z1vjWItUBSmUmV2SXryjbKj5-MVQ/view?usp=sharing

The screencast shows:

• App.svelte routing mechanism works
• gql in LIFF works with GraphQL context & @auth mechanism
• Production Cache-control: max-age works

Fresh LIFF button

For a new LIFF button, LIFF HTML is fetched from server, which makes the LIFF redirection obvious
https://drive.google.com/file/d/1i0pl1iqHaMVNTJR6cm8_mpf6SHK5A7qw/view?usp=sharing

The screencast shows:

• 'loading' state works
• LIFF redirection handling works (Green loading bar appears twice)

Abort LIFF for buttons in previous search sessions

When user inputs a new rumor, it is considered as "creating a new search session". LIFF opened with previous sessions are aborted.

https://drive.google.com/file/d/1iUmdNL4os9yyTPHOqyLUjWOl2Ewb7HMR/view?usp=sharing

[coveralls]

Pull Request Test Coverage Report for Build 738

• 10 of 10 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 98.636%

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Totals
Change from base Build 714:	0.03%
Covered Lines:	302
Relevant Lines:	303

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💛 - Coveralls