adding feature_threshold replacement for show_all_features

[bchen1116]

fix #869

Replace show_all_features parameter in graph_permutation_importance() and graph_feature_importance() functions

Creating parameter feature_threshold to replace show_all_features for graphing feature importance.

[codecov]

Codecov Report

Merging #1097 into main will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1097   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files         192      192           
  Lines       10719    10736   +17     
=======================================
+ Hits        10710    10727   +17     
  Misses          9        9           

Impacted Files	Coverage Δ
evalml/model_understanding/graphs.py	100.00% <100.00%> (ø)
evalml/pipelines/pipeline_base.py	100.00% <100.00%> (ø)
...lml/tests/model_understanding_tests/test_graphs.py	100.00% <100.00%> (ø)
evalml/tests/pipeline_tests/test_graphs.py	100.00% <100.00%> (ø)

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[dsherry]

@bchen1116 great work!! Nice unit tests.

I have a few requests before you merge:

• I left a suggestion about renaming the parameter to importance_threshold
• Remove unnecessary threshold values from a few of the tests, or explain why they're necessary
• Please see my comment about updating the two new unit tests you've added.
• Delete the file evalml/tests/utils_tests/test_graph_utils.py which was added accidentally (was recently deleted on main)

I left some other nit-pick comments on style and wording, but those aren't blocking merge and could be addressed in a separate PR if you prefer.

[angela97lin]

I think Dylan covered the major points but looks good after those comments are addressed!