Make Datepicker and Timepicker more consistent

[CGA1123]

• Rename DatePicker to Datepicker
• Remove Timepicker#initial_time + Timepicker#placeholder, inject them in initializer instead.
• Rename #date_picker to #datepicker
• Add #timepicker helpers

[codecov[bot]]

Codecov Report

Merging #145 (89288b1) into master (1968d75) will increase coverage by 0.24%. The diff coverage is 95.45%.

:exclamation: Current head 89288b1 differs from pull request most recent head 96dc8e7. Consider uploading reports for the commit 96dc8e7 to get more accurate results

@@            Coverage Diff             @@
##           master     #145      +/-   ##
==========================================
+ Coverage   99.40%   99.64%   +0.24%     
==========================================
  Files          40       40              
  Lines         837      842       +5     
==========================================
+ Hits          832      839       +7     
+ Misses          5        3       -2     

Impacted Files	Coverage Δ
lib/slack/block_kit/layout/actions.rb	94.54% <83.33%> (-5.46%)	:arrow_down:
lib/slack/block_kit/element/datepicker.rb	100.00% <100.00%> (ø)
lib/slack/block_kit/element/timepicker.rb	100.00% <100.00%> (ø)
lib/slack/block_kit/layout/input.rb	100.00% <100.00%> (+6.57%)	:arrow_up:
lib/slack/block_kit/layout/section.rb	100.00% <100.00%> (ø)

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