Fix context.views.open in slack home tab

[etrex]

previous implement

• Set channelId to view.privateMetadata to store the session key in the view.
• If a webhook event from button clicked on a modal, get sessionKey from view.privateMetadata.

current implement

• Get sessionKey from channelId or userId.
• If a webhook event from button clicked on a channel modal, get sessionKey from view.privateMetadata.
• If a webhook event from button clicked on a home modal, get sessionKey from user id.

[codecov-commenter]

Codecov Report

Merging #809 into release/1.5 will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           release/1.5     #809   +/-   ##
============================================
  Coverage        83.93%   83.93%           
============================================
  Files              120      120           
  Lines             6167     6169    +2     
  Branches          1523     1525    +2     
============================================
+ Hits              5176     5178    +2     
  Misses             987      987           
  Partials             4        4           

Impacted Files	Coverage Δ
packages/bottender/src/slack/SlackTypes.ts	100.00% <ø> (ø)
packages/bottender/src/slack/SlackConnector.ts	91.56% <100.00%> (+0.10%)	⬆️
packages/bottender/src/slack/SlackContext.ts	92.30% <100.00%> (+0.09%)	⬆️
packages/bottender-facebook/src/FacebookContext.ts	74.39% <0.00%> (-0.31%)	⬇️

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