[bug] fix shared explore url

[graceguo-supercat]

Shared explore url was broken since #4490.
for user shared slice url: /superset/explore?r=123, there is no request or post params. need to add logic in get_form_data function, to get saved request params from url table.

@mistercrunch

[codecov-io]

Codecov Report

Merging #4560 into master will decrease coverage by <.01%.
The diff coverage is 25%.

@@            Coverage Diff            @@
##           master   #4560      +/-   ##
=========================================
- Coverage    71.2%   71.2%   -0.01%     
=========================================
  Files         187     187              
  Lines       14786   14785       -1     
  Branches     1083    1083              
=========================================
- Hits        10528   10527       -1     
  Misses       4255    4255              
  Partials        3       3

Impacted Files	Coverage Δ
superset/views/core.py	71.15% <25%> (-0.03%)	⬇️

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[michellethomas]

lgtm