fix(create-bottender-app): fix context concat and env name

[chentsulin]

• replace SLACK_VERIFICATION_TOKEN to SLACK_SIGNING_SECRET

[codecov-commenter]

Codecov Report

Merging #859 into master will decrease coverage by 0.04%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master     #859      +/-   ##
==========================================
- Coverage   83.27%   83.23%   -0.05%     
==========================================
  Files         108      108              
  Lines        5644     5647       +3     
  Branches     1365     1422      +57     
==========================================
  Hits         4700     4700              
+ Misses        921      919       -2     
- Partials       23       28       +5     

Impacted Files	Coverage Δ
packages/bottender/src/server/Server.ts	8.69% <0.00%> (-0.30%)	⬇️
packages/bottender/src/slack/SlackEvent.ts	93.84% <ø> (ø)
packages/bottender/src/slack/routes.ts	100.00% <ø> (ø)
...create-bottender-app/src/utils/generateAppEntry.ts	0.00% <ø> (ø)
packages/bottender-dialogflow/src/index.ts	95.45% <100.00%> (ø)
...ackages/bottender/src/session/MongoSessionStore.ts	97.82% <100.00%> (ø)
packages/bottender/src/getClient.ts	52.63% <0.00%> (ø)
packages/create-bottender-app/src/shared/log.ts	0.00% <0.00%> (ø)

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